with whom Justice Marshall, Justice Powell, and Justice Stevens join, dissenting.
The Court today holds that an incumbent teachers’ union may negotiate a collective-bargaining agreement with a school board that grants the incumbent access to teachers’ *56mailboxes and to the interschool mail system and denies such access to a rival union. Because the exclusive-access provision in the collective-bargaining agreement amounts to viewpoint discrimination that infringes the respondents’ First Amendment rights and fails to advance any substantial state interest, I dissent.1
I
The Court properly acknowledges that teachers have protected First Amendment rights within the school context. See Tinker v. Des Moines School District, 393 U. S. 503, 506 (1969). In particular, we have held that teachers may not be “compelled to relinquish the First Amendment rights they would otherwise enjoy as citizens to comment on matters of public interest in connection with the operation of the public schools in which they work.” Pickering v. Board of Education, 391 U. S. 563, 568 (1968). See also Mt. Healthy City Board of Education v. Doyle, 429 U. S. 274, 284 (1977). We also have recognized in the school context the First Amendment right of “individuals to associate to further their personal beliefs,” Healy v. James, 408 U. S. 169, 181 (1972), and have acknowledged the First Amendment rights of dissident teachers in matters involving labor relations. City of Madison Joint School District v. Wisconsin Employment Relations Comm’n, 429 U. S. 167, 176, n. 10 (1976). Against this background it is clear that the exclusive-access policy in this case implicated the respondents’ First Amendment rights by restricting their freedom of expression on issues important to the operation of the school system. As the Court of Appeals suggested, this speech is “if not at the very apex of any hierarchy of protected speech, at least not far below it.” Perry Local Educators’ Assn. v. Hohlt, 652 F. 2d 1286, 1299 (CA7 1981).
From this point of departure the Court veers sharply off course. Based on a finding that the interschool mail system *57is not a “public forum,” ante, at 48-49, the Court states that the respondents have no right of access to the system, ibid., and that the School Board is free “to make distinctions in access on the basis of subject matter and speaker identity,” ante, at 49, if the distinctions are “reasonable in light of the purpose which the forum at issue serves.” Ibid, (footnote omitted). According to the Court, the petitioner’s status as the exclusive bargaining representative provides a reasonable basis for the exclusive-access policy.
The Court fundamentally misperceives the essence of the respondents’ claims and misunderstands the thrust of the Court of Appeals’ well-reasoned opinion. This case does not involve an “absolute access” claim. It involves an “equal access” claim. As such it does not turn on whether the internal school mail system is a “public forum.” In focusing on the public forum issue, the Court disregards the First Amendment’s central proscription against censorship, in the form of viewpoint discrimination, in any forum, public or nonpublic.
A
The First Amendment’s prohibition against government discrimination among viewpoints on particular issues falling within the realm of protected speech has been noted extensively in the opinions of this Court. In Niemotko v. Maryland, 340 U. S. 268 (1951), two Jehovah’s Witnesses were denied access to a public park to give Bible talks. Members of other religious organizations had been granted access to the park for purposes related to religion. The Court found that the denial of access was based on public officials’ disagreement with the Jehovah’s Witnesses’ views, id., at 272, and held it invalid. During the course of its opinion, the Court stated: “The right to equal protection of the laws, in the exercise of those freedoms of speech and religion protected by the First and Fourteenth Amendments, has a firmer foundation than the whims or personal opinions of a local governing body.” Ibid. In an opinion concurring in the result, Justice *58Frankfurter stated that “[t]o allow expression of religious views by some and deny the same privilege to others merely because they or their views are unpopular, even deeply so, is a denial of equal protection of the law forbidden by the Fourteenth Amendment.” Id., at 284. See also Fowler v. Rhode Island, 345 U. S. 67, 69 (1953).
In Tinker v. Des Moines School District, supra, we held unconstitutional a decision by school officials to suspend students for wearing black armbands in protest of the war in Vietnam. The record disclosed that school officials had permitted students to wear other symbols relating to politically significant issues. Id., at 510. The black armbands, however, as symbols of opposition to the Vietnam War, had been singled out for prohibition. We stated: “Clearly, the prohibition of expression of one particular opinion, at least without evidence that it is necessary to avoid material and substantial interference with schoolwork or discipline, is not constitutionally permissible.” Id., at 511.
City of Madison Joint School District v. Wisconsin Employment Relations Comm’n, supra, considered the question of whether a State may constitutionally require a board of education to prohibit teachers other than union representatives from speaking at public meetings about matters relating to pending collective-bargaining negotiations. The board had been found guilty of a prohibited labor practice for permitting a teacher to speak who opposed one of the proposals advanced by the union in contract negotiations. The board was ordered to cease and desist from permitting employees, other than union representatives, to appear and to speak at board meetings on matters subject to collective bargaining. We held this order invalid. During the course of our opinion we stated: “Whatever its duties as an employer, when the board sits in public meetings to conduct public business and hear the views of citizens, it may not be required to discriminate between speakers on the basis of their employment, or the content of their speech. See Police Dept. of *59Chicago v. Mosley, 408 U. S. 92, 96 (1972).” 429 U. S., at 176 (footnote omitted).2
There is another line of cases, closely related to those implicating the prohibition against viewpoint discrimination, that have addressed the First Amendment principle of subject-matter, or content neutrality. Generally, the concept of content neutrality prohibits the government from choosing the subjects that are appropriate for public discussion. The content-neutrality cases frequently refer to the prohibition against viewpoint discrimination and both concepts have their roots in the First Amendment’s bar against censorship. But unlike the viewpoint-discrimination concept, which is used to strike down government restrictions on speech by particular speakers, the content-neutrality principle is invoked when the government has imposed restrictions on speech related to an entire subject area. The content-neutrality principle can be seen as an outgrowth of the core First Amendment prohibition against viewpoint discrimination. See generally Stone, Restrictions of Speech Because of its Content: The Peculiar Case of Subject-Matter Restrictions, 46 U. Chi. L. Rev. 81 (1978).
*60We have invoked the prohibition against content discrimination to invalidate government restrictions on access to public forums. See, e. g., Carey v. Brown, 447 U. S. 455 (1980); Grayned v. City of Rockford, 408 U. S. 104 (1972); Police Department of Chicago v. Mosley, 408 U. S. 92 (1972). We also have relied on this prohibition to strike down restrictions on access to a limited public forum. See, e. g., Widmar v. Vincent, 454 U. S. 263 (1981). Finally, we have applied the doctrine of content neutrality to government regulation of protected speech in cases in which no restriction of access to public property was involved. See, e. g., Consolidated Edison Co. v. Public Service Comm’n, 447 U. S. 530 (1980); Erznoznik v. City of Jacksonville, 422 U. S. 205 (1975). See also Metromedia, Inc. v. San Diego, 453 U. S. 490, 513, 515, 516 (1981) (plurality opinion).
Admittedly, this Court has not always required content neutrality in restrictions on access to government property. We upheld content-based exclusions in Lehman v. City of Shaker Heights, 418 U. S. 298 (1974), in Greer v. Spock, 424 U. S. 828 (1976), and in Jones v. North Carolina Prisoners’ Union, 433 U. S. 119 (1977). All three cases involved an unusual forum, which was found to be nonpublic, and the speech was determined for a variety of reasons to be incompatible with the forum. These cases provide some support for the notion that the government is permitted to exclude certain subjects from discussion in nonpublic forums.3 They pro*61vide no support, however, for the notion that government, once it has opened up government property for discussion of specific subjects, may discriminate among viewpoints on those topics. Although Greer, Lehman, and Jones permitted content-based restrictions, none of the cases involved viewpoint discrimination. All of the restrictions were viewpoint-neutral. We expressly noted in Greer that the exclusion was “objectively and evenhandedly applied.” 424 U. S., at 839.4
Once the government permits discussion of certain subject matter, it may not impose restrictions that discriminate among viewpoints on those subjects whether a nonpublic forum is involved or not.5 This prohibition is implicit in the Mosley line of cases, in Tinker v. Des Moines School District, 393 U. S. 503 (1969), and in those cases in which we have approved content-based restrictions on access to government property that is not a public forum. We have never held that government may allow discussion of a sub*62ject and then discriminate among viewpoints on that particular topic, even if the government for certain reasons may entirely exclude discussion of the subject from the forum. In this context, the greater power does not include the lesser because for First Amendment purposes exercise of the lesser power is more threatening to core values. Viewpoint discrimination is censorship in its purest form and government regulation that discriminates among viewpoints threatens the continued vitality of “free speech.”
B
Against this background, it is clear that the Court’s approach to this case is flawed. By focusing on whether the interschool mail system is a public forum, the Court disregards the independent First Amendment protection afforded by the prohibition against viewpoint discrimination.6 This *63case does not involve a claim of an absolute right of access to the forum to discuss any subject whatever. If it did, public forum analysis might be relevant. This case involves a claim of equal access to discuss a subject that the Board has approved for discussion in the forum. In essence, the respondents are not asserting a right of access at all; they are asserting a right to be free from discrimination. The critical inquiry, therefore, is whether the Board’s grant of exclusive access to the petitioner amounts to prohibited viewpoint discrimination.
II
The Court addresses only briefly the respondents’ claim that the exclusive-access provision amounts to viewpoint discrimination. In rejecting this claim, the Court starts from the premise that the school mail system is not a public forum7 and that, as a result, the Board has no obligation to *64grant access to the respondents. The Court then suggests that there is no indication that the Board intended to discourage one viewpoint and to advance another. In the Court’s view, the exclusive-access policy is based on the status of the respective parties rather than on their views. The Court then states that “[i]mplicit in the concept of the nonpublic forum is the right to make distinctions in access on the basis of subject matter and speaker identity.” Ante, at 49. According to the Court, “[t]hese distinctions may be impermissible in a public forum but are inherent and inescapable in the process of limiting a nonpublic forum to activities compatible with the intended purpose of the property.” Ibid.
As noted, whether the school mail system is a public forum or not the Board is prohibited from discriminating among viewpoints on particular subjects. Moreover, whatever the right of public authorities to impose content-based restrictions on access to government property that is a nonpublic forum,8 once access is granted to one speaker to discuss a certain subject access may not be denied to another speaker based on his viewpoint. Regardless of the nature of the forum, the critical inquiry is whether the Board has engaged in prohibited viewpoint discrimination.
The Court responds to the allegation of viewpoint discrimination by suggesting that there is no indication that the Board intended to discriminate and that the exclusive-access policy is based on the parties’ status rather than on their views. In this case, for the reasons discussed below, see infra, at 66-71, the intent to discriminate can be inferred from the effect of the policy, which is to deny an effective channel of communication to the respondents, and from other *65facts in the case. In addition, the petitioner’s status has nothing to do with whether viewpoint discrimination in fact has occurred. If anything, the petitioner’s status is relevant to the question of whether the exclusive-access policy can be justified, not to whether the Board has discriminated among viewpoints. See infra, at 66-69.
Addressing the question of viewpoint discrimination directly, free of the Court’s irrelevant public forum analysis, it is clear that the exclusive-access policy discriminates on the basis of viewpoint. The Court of Appeals found that “[t]he access policy adopted by the Perry schools, in form a speaker restriction, favors a particular viewpoint on labor relations in the Perry schools . . . : the teachers inevitably will receive from [the petitioner] self-laudatory descriptions of its activities on their behalf and will be denied the critical perspective offered by [the respondents].” Perry Local Educators’ Assn. v. Hohlt, 652 F. 2d, at 1296. This assessment of the effect of the policy is eminently reasonable. Moreover, certain other factors strongly suggest that the policy discriminates among viewpoints.
On a practical level, the only reason for the petitioner to seek an exclusive-access policy is to deny its rivals access to an effective channel of communication. No other group is explicitly denied access to the mail system. In fact, as the Court points out, ante, at 47-48, many other groups have been granted access to the system. Apparently, access is denied to the respondents because of the likelihood of their expressing points of view different from the petitioner’s on a range of subjects. The very argument the petitioner advances in support of the policy, the need to preserve labor peace, also indicates that the access policy is not viewpoint-neutral.
In short, the exclusive-access policy discriminates against the respondents based on their viewpoint. The Board has agreed to amplify the speech of the petitioner, while repressing the speech of the respondents based on the respondents’ point of view. This sort of discrimination amounts to censor*66ship and infringes the First Amendment rights of the respondents. In this light, the policy can survive only if the petitioner can justify it.
Ill
In assessing the validity of the exclusive-access policy, the Court of Appeals subjected it to rigorous scrutiny. Perry Local Educators’ Assn. v. Hohlt, supra, at 1296. The court pursued this course after a careful review of our cases and a determination that “no case has applied any but the most exacting scrutiny to a content or speaker restriction that substantially tended to favor the advocacy of one point of view on a given issue.” 652 F. 2d, at 1296. The Court of Appeals’ analysis is persuasive. In light of the fact that viewpoint discrimination implicates core First Amendment values, the exclusive-access policy can be sustained “only if the government can show that the regulation is a precisely drawn means of serving a compelling state interest.” Consolidated Edison Co. v. Public Service Comm’n, 447 U. S., at 540. Cf. Carey v. Brown, 447 U. S., at 461-462 (to be valid, legislation must be “finely tailored to serve substantial state interests, and the justifications offered for any distinctions it draws must be carefully scrutinized”); Police Department of Chicago v. Mosley, 408 U. S., at 98-99 (discrimi-nations “must be tailored to serve a substantial governmental interest”).
A
The petitioner attempts to justify the exclusive-access provision based on its status as the exclusive bargaining representative for the teachers and on the State’s interest in efficient communication between collective-bargaining representatives and the members of the unit. The petitioner’s status and the State’s interest in efficient communication are important considerations. They are not sufficient, however, to sustain the exclusive-access policy.
As the Court of Appeals pointed out, the exclusive-access policy is both “overinclusive and underinclusive” as a means *67of serving the State’s interest in the efficient discharge of the petitioner’s legal duties to the teachers. Perry Local Educators’ Assn. v. Hohlt, 652 F. 2d, at 1300. The policy is overinclusive because it does not strictly limit the petitioner’s use of the mail system to performance of its special legal duties and underinclusive because the Board permits outside organizations with no special duties to the teachers, or to the students, to use the system. Ibid. The Court of Appeals also suggested that even if the Board had attempted to tailor the policy more carefully by denying outside groups access to the system and by expressly limiting the petitioner’s use of the system to messages relating to its official duties, “the fit would still be questionable, for it might be difficult — both in practice and in principle — effectively to separate ‘necessary’ communications from propaganda.” Ibid. The Court of Appeals was justly concerned with this problem, because the scope of the petitioner’s “legal duties” might be difficult, if not impossible, to define with precision. In this regard, we alluded to the potential scope of collective-bargaining responsibilities in City of Madison Joint School District v. Wisconsin Employment Relations Comm’n, 429 U. S. 167 (1976), when we stated: “[T]here is virtually no subject concerning the operation of the school system that could not also be characterized as a potential subject of collective bargaining.” Id., at 177.9
*68Putting aside the difficulties with the fit between this policy and the asserted interests, the Court of Appeals properly pointed out that the policy is invalid “because it furthers no discernible state interest.” Perry Local Educators’ Assn. v. Hohlt, 652 F. 2d, at 1300. While the Board may have a legitimate interest in granting the petitioner access to the system, it has no legitimate interest in making that access exclusive by denying access to the respondents. As the Court of Appeals stated: “Without an independent reason why equal access for other labor groups and individual teachers is unde*69sirable, the special duties of the incumbent do not justify opening the system to the incumbent alone.” Ibid. In this case, for the reasons discussed below, there is no independent reason for denying access to the respondents.10
*70B
The petitioner also argues, and the Court agrees, ante, at 52, that the exclusive-access policy is justified by the State’s interest in preserving labor peace. As the Court of Appeals found, there is no evidence on this record that granting access to the respondents would result in labor instability. 652 F. 2d, at 1301.11 In addition, there is no reason to assume that the respondents’ messages would be any more likely to cause labor discord when received by members of the majority union than the petitioner’s messages would when received by the respondents. Moreover, it is noteworthy that both the petitioner and the respondents had access to the mail system for some time prior to the representation election. See ante, at 39. There is no indication that this policy resulted in disruption of the school environment.12
*71Although the State’s interest in preserving labor peace in the schools in order to prevent disruption is unquestionably substantial, merely articulating the interest is not enough to sustain the exclusive-access policy in this case. There must be some showing that the asserted interest is advanced by the policy. In the absence of such a showing, the exclusive-access policy must fall.13
C
Because the grant to the petitioner of exclusive access to the internal school mail system amounts to viewpoint discrimination that infringes the respondents’ First Amendment rights and because the petitioner has failed to show that the policy furthers any substantial state interest, the policy must be invalidated as violative of the First Amendment.
IY
In order to secure the First Amendment’s guarantee of freedom of speech and to prevent distortions of “the marketplace of ideas,” see Abrams v. United States, 250 U. S. 616, 630 (1919) (Holmes, J., dissenting), governments generally are prohibited from discriminating among viewpoints on is*72sues within the realm of protected speech. In this case the Board has infringed the respondents’ First Amendment rights by granting exclusive access to an effective channel of communication to the petitioner and denying such access to the respondents. In view of the petitioner’s failure to establish even a substantial state interest that is advanced by the exclusive-access policy, the policy must be held to be constitutionally infirm. The decision of the Court of Appeals should be affirmed.
1 agree with the Court’s conclusion that the appeal should be dismissed for want of appellate jurisdiction. See ante, at 43.
See also Widmar v. Vincent, 454 U. S. 263, 280 (1981) (Stevens, J., concurring in judgment) (“[T]he university . . . may not allow its agreement or disagreement with the viewpoint of a particular speaker to determine whether access to a forum will be granted. If a state university is to deny recognition to a student organization — or is to give it a lesser right to use school facilities than other student groups — it must have a valid reason for doing so”); First National Bank of Boston v. Bellotti, 435 U. S. 765, 784-786 (1978) (“In the realm of protected speech, the legislature is constitutionally disqualified from dictating the subjects about which persons may speak and the speakers who may address a public issue. . . . Especially where, as here, the legislature’s suppression of speech suggests an attempt to give one side of a debatable public question an advantage in expressing its views to the people, the First Amendment is plainly offended” (citation omitted) (footnote omitted)); Healy v. James, 408 U. S. 169, 187-188 (1972) (the State “may not restrict speech or association simply because it finds the views expressed by any group to be abhorrent”).
There are several factors suggesting that these decisions are narrow and of limited importance. First, the forums involved were unusual. A military base was involved in Greer v. Spock, advertising space on a city transit system in Lehman v. City of Shaker Heights, and a prison in Jones v. North Carolina Prisoners’ Union. Moreover, the speech involved was arguably incompatible with each forum, especially in Greer, which involved speeches and demonstrations of a partisan political nature on a military base, and in Jones, which involved labor union organizational activities in a prison. Finally, we have noted the limited scope of Greer and Lehman in subsequent opinions. See, e. g., Consolidated Edison Co. v. Public Service Comm’n, 447 U. S. 530, 539-540 (1980); Metromedia, Inc. v. *61San Diego, 453 U. S. 490, 514, n. 19 (1981) (plurality opinion); Erznoznik v. City of Jacksonville, 422 U. S. 205, 209 (1975).
In his concurring opinion in Greer v. Spock, Justice Powell noted the absence of any viewpoint discrimination in the regulations and stated that the military authorities would be barred from discriminating among viewpoints on political issues. 424 U. S., at 848, n. 3.
In other cases in which we have upheld restrictions on access to government property, the restrictions have been both content- and viewpoint-neutral. See, e. g., United States Postal Service v. Council of Greenburgh Civic Assns., 453 U. S. 114 (1981); Adderley v. Florida, 385 U. S. 39 (1966).
This is not to suggest that a government may not close a nonpublie forum altogether or limit access to the forum to those involved in the “official business” of the agency. Restrictions of this type are consistent with the government’s right “ ‘to preserve the property under its control for the use to which it is lawfully dedicated.’ ” Ante, at 46 (quoting United States Postal Service v. Council of Greenburgh Civic Assns., supra, at 129-130). Limiting access to a nonpublic government forum to those involved in the “official business” of the agency also protects the interest of the government, qua government, in speaking clearly and definitively.
Lower courts have recognized that the prohibition against viewpoint discrimination affords speakers protection independent of the public forum doctrine. See, e. g., National Black United Fund, Inc. v. Devine, 215 U. S. App. D. C. 130, 136, 667 F. 2d 173, 179 (1981); Jaffe v. Alexis, 659 F. 2d 1018, 1020-1021, n. 2 (CA9 1981); Bonner-Lyons v. School Committee of City of Boston, 480 F. 2d 442, 444 (CA1 1973). In Jaffe, the Ninth Circuit stated: “When the content of the speaker’s message forms the basis for its selective regulation, public forum analysis is no longer crucial; the government must still justify the restriction and the justification ‘must be scrutinized more carefully to ensure that communication has not been prohibited “merely because public officials disapprove of the speaker’s views.”’” 659 F. 2d, at 1020-1021, n. 2 (citations omitted). See also United States Postal Service v. Council ofGreenburgh Civic Assns., supra, at 136,140 (Brennan, J., concurring in judgment).
In Greer v. Spock, supra, I suggested that an undue focus on public forum issues can blind the Court to proper regard for First Amendment interests. After noting that “the notion of ‘public forum’ has never been the touchstone of public expression,” id., at 859 (dissenting opinion), I stated:
“Those cases permitting public expression without characterizing the locale involved as a public forum, together with those cases recognizing the existence of a public forum, albeit qualifiedly, evidence the desirability of a flexible approach to determining whether public expression should be pro*63tected. Realizing that the permissibility of a certain form of public expression at a given locale may differ depending on whether it is asked if the locale is a public forum or if the form of expression is compatible with the activities occurring at the locale, it becomes apparent that there is need for a flexible approach. Otherwise, with the rigid characterization of a given locale as not a public forum, there is the danger that certain forms of public speech at the locale may be suppressed, even though they are basically compatible with the activities otherwise occurring at the locale.” Id., at 859-860.
It is arguable that the school mail system could qualify for treatment as a public forum of some description if one focuses on whether “ ‘the manner of expression is basically incompatible with the normal activity of a particular place at a particular time.’ Grayned v. City of Rockford, [408 U. S.], at 116.” United States Postal Service v. Council of Greenburgh Civic Associations, 453 U. S., at 136 (Brennan, J., concurring in judgment). It is difficult to see how granting the respondents access to the mailboxes would be incompatible with the normal activities of the school especially in view of the fact that the petitioner and outside groups enjoy such access. The petitioner’s messages, and certainly those of the outside groups, do not appear to be any more compatible with the normal activity of the school than the respondents’ messages would be. It is not necessary to reach this issue, however, in view of the existence of impermissible viewpoint discrimination.
The Court’s reference to the government’s right to make distinctions in access based on “speaker identity” might be construed as a reference to the government’s interest in restricting access to a nonpublic forum to those involved in the “official business” of the particular agency. See n. 5, supra. The “speaker identity” distinction in this case, however, cannot be justified on this basis. See n. 10, infra.
The Court rejects the Court of Appeals’ finding that the exclusive-access policy was overinclusive on the ground that “the record . . . does not establish that [the petitioner] enjoyed or claimed unlimited access by usage or otherwise; indeed, the collective-bargaining agreement indicates that the right of access was accorded to [the petitioner] ‘acting as the representative of the teachers.’” Ante, at 53, n. 13. Under these circumstances, the Court suggests that it is unnecessary “to decide the reasonableness of a grant of access for unlimited purposes.” Ibid. This argument is flawed in three ways. First, the Court of Appeals found that “the collective bargaining agreement [did] not limit [the petitioner’s] use of the mail system to messages related to its special legal duties,” Perry Local Educators’ *68Assn. v. Hohlt, 652 F. 2d 1286, 1300 (CA71981), and there is nothing in the record to indicate that the petitioner did not enjoy unlimited access. Second, we noted above the nearly limitless scope of collective-bargaining responsibilities. See supra, at 67. With no apparent monitoring of the petitioner’s messages by the board, Perry Local Educators’ Assn. v. Hohlt, supra, at 1293, n. 29, it is clear that there is no real limit to the petitioner’s “special legal duties.” Finally, even assuming that the Board had a narrowly tailored policy that expressly limited the petitioner’s access to official messages and included school monitoring of the messages, it still would be difficult, as the Court of Appeals pointed out, “to separate ‘necessary’ communications from propaganda.” 652 F. 2d, at 1300.
The Court rejects the Court of Appeals’ determination that the policy was underinclusive on the ground that there was no District policy of “open access for private groups and, in any event, the provision of access to these private groups does not undermine the reasons for not allowing similar access by a rival labor union.” Ante, at 53, n. 13 (citing Greer v. Spock, 424 U. S., at 838, n. 10). Even though there was no apparent policy of open access, the provision of access to outside groups certainly undermines the petitioner’s asserted justification for the policy and establishes that the policy is overinclusive with respect to that justification. Moreover, if all unions were denied access to the mail system, there might be some force to the Court’s reliance on Greer for the notion that granting access to some groups does not undermine the reasons for denying it to others. But in a case where the government grants access to one labor group, and denies it to another, Greer is irrelevant because even read broadly Greer does not support a right on the part of the government to discriminate among viewpoints on subjects approved for discussion in the forum. See supra, at 60-61.
A variant of the “special legal duties” justification for the exclusive-access policy is the “official business” justification. As noted, see n. 5, swpra, the government has a legitimate interest in limiting access to a nonpublic forum to those involved in the “official business” of the agency. This interest may justify restrictions based on speaker identity, as for example, when a school board denies access to a classroom to persons other than teachers. Such a speaker identity restriction may have a viewpoint discriminatory effect, but it is justified by the government’s interest in clear, definitive classroom instruction.
In this case, an “official business” argument is inadequate to justify the exclusive-access policy for many of the same reasons that the “special legal duties” rationale is inadequate. As with its relation to the “special legal duties” argument, the exclusive-access policy is both overinclusive and underinclusive with respect to an “official business” justification. First, as the Court of Appeals pointed out, the School Board neither monitors nor endorses the petitioner’s messages. Perry Local Educators’ Assn. v. Hohlt, supra, at 1293, n. 29. In this light, it is difficult to consider the petitioner an agent of the Board. Moreover, in light of the virtually unlimited scope of a union’s collective-bargaining duties, it expands the definition of “official business” beyond any clear meaning to suggest that the petitioner’s messages are always related to the school system’s “official business.”
More importantly, however, the only Board policy discernible from this record involves a denial of access to one group: the respondents. The Board has made no explicit effort to restrict access to those involved in the “official business” of the schools. In fact, access has been granted to outside groups such as parochial schools, church groups, YMCA’s, and Cub Scout units. See ante, at 47-48. It is difficult to discern how these groups are involved in the “official business” of the school. The provision of access to these groups strongly suggests that the denial of access to the respondents was not based on any desire to limit access to the forum to those involved in the “official business” of the schools; instead, it suggests that it was based on hostility to the point of view likely to be expressed by the respondents. The Board simply has agreed to shut out one voice on a subject approved for discussion in the forum. This is impermissible.
The Court suggests that proof of disruption is not necessary “to justify the denial of access to a nonpublic forum on grounds that the proposed use may disrupt the property’s intended function,” ante, at 52, n. 12, and again cites Greer v. Spock, supra. In Tinker v. Des Moines School District, 393 U. S. 503 (1969), which is discussed supra, at 58, we noted that “in our system, undifferentiated fear or apprehension of disturbance is not enough to overcome the right to freedom of expression.” 393 U. S., at 508. Later, we stated that “where there is no finding and no showing that engaging in the forbidden conduct would ‘materially and substantially interfere with the requirements of appropriate discipline in the operation of the school,’ the prohibition cannot be sustained.” Id., at 509 (citation omitted). Finally, we stated that “the prohibition of expression of one particular opinion, at least without evidence that it is necessary to avoid material and substantial interference with schoolwork or discipline, is not constitutionally permissible.” Id., at 511. It is noteworthy that Tinker involved what the Court would be likely to describe as a nonpublic forum. See also City of Madison Joint School District v. Wisconsin Employment Relations Comm’n, 429 U. S. 167, 173-174 (1976); Healy v. James, 408 U. S., at 190-191. These cases establish that the State must offer evidence to support an allegation of potential disruption in order to sustain a restriction on protected speech.
It appears, therefore, that the exclusive-access provision was included solely at the demand of the majority union in collective-bargaining negotia*71tions. We note that, in this case, the School Board did not even seek review of the Court of Appeals’ holding that the mailboxes and the inter-school mail system must be open to both unions.
The Court also cites the availability of alternative channels of communication in support of the “reasonableness” of the exclusive-access policy. Ante, at 53. In a detailed discussion, the Court of Appeals properly concluded that the other channels of communication available to the respondents were “not nearly as effective as the internal mail system.” Perry Local Educators’ Assn. v. Hohlt, 652 F. 2d, at 1299. See also id., at 1299-1300. In addition, the Court apparently disregards the principle that “one is not to have the exercise of his liberty of expression in appropriate places abridged on the plea that it may be exercised in some other place.” Schneider v. State, 308 U. S. 147, 163 (1939). In this case, the existence of inferior alternative channels of communication does not affect the conclusion that the petitioner has failed to justify the viewpoint-discriminatory exclusive-access policy.