*752OPINION
LÓPEZ, Justice.This appeal originated from a personal injury lawsuit filed by William and Linda Foster to recover damages for injuries sustained by their son, William Foster, Jr. William, Jr. was injured when he climbed up the wall in his school’s gymnasium to retrieve a tennis ball that was stuck between the wall and a metal rail. In attempting to retrieve the ball, William, Jr. fell and was injured. To recover damages resulting from their son’s injuries, the Fosters sued the school’s principal, Noel Estrada, and the school’s coach, Joe A. Gonzalez.
Estrada and Gonzalez moved for summary judgment in response to the Fosters’ lawsuit. In their motion for summary judgment, Estrada and Gonzalez asserted the affirmative defense provided in section 22.051 of the Texas Education Code. To support their interpretation of section 22.051, Estrada and Gonzalez relied on Barr v. Bernhard, 562 S.W.2d 844 (Tex.1978), an opinion in which the Supreme Court of Texas interpreted the predecessor to section 22.051 — section 21.912. In response, the trial court granted summary judgment in favor of Estrada and Gonzalez.
On appeal, the Fosters raise three issues to challenge the trial court’s order granting summary judgment in favor of Estrada and Gonzalez: (1) the trial court erred in concluding that defendants’ omissions were discretionary as a matter of law and thus summary judgment was improper; (2) because the defendants’ summary judgment evidence was conclusory and therefore insufficient as a matter of law, the trial court erred in granting the summary judgment; and (3) because defendants’ summary judgment evidence was not readily controvertible and therefore insufficient as a matter of law, the trial court erred in granting the summary judgment. Based on these errors, the Fosters ask us to reverse the summary judgment.
Standard of Review
The party presenting the motion for summary judgment has the burden of showing that no genuine issue of material fact exists and that it is entitled to judgment as a matter of law. See Tex.R. Civ. P. 166a(c); Nixon v. Mr. Property Management Co., 690 S.W.2d 546, 548 (Tex.1985); Swilley v. Hughes, 488 S.W.2d 64, 67 (Tex.1972). In deciding whether a disputed material fact issue exists, thereby excluding summary judgment, the reviewing court will take as true all evidence favoring the non-movant. See Nixon, 690 S.W.2d at 548-49; Montgomery v. Kennedy, 669 S.W.2d 309, 311 (Tex.1984). All reasonable inferences from the evidence will be weighed in favor of the non-movant, and any doubts will be resolved in the non-movant’s favor. See Nixon, 690 S.W.2d at 549; Montgomery, 669 S.W.2d at 311.
A motion for summary judgment must expressly present the grounds upon which it is made. See McConnell v. Southside School Dist., 858 S.W.2d 337, 341 (Tex.1993). In the instant case, Estrada and Gonzalez moved for summary judgment on the grounds of an affirmative defense. When a defendant moves for summary judgment based on an affirmative defense, the defendant must prove conclusively all elements of the affirmative defense as a matter of law, such that there is no issue of material fact to be resolved by the trier of fact. See Montgomery, 669 S.W.2d at 310-11.
The Affirmative Defense Provided in the Education Code
In their motion for summary judgment, Estrada and Gonzalez relied on section 22.051 of the Education Code. Section 22.051 provides:
A professional employee1 of a school district is not personally liable for any act that is incident to or within the scope of the duties of the employee’s position of employment and that involves the exercise *753of judgment or discretion on the part of the employee, except in circumstances in which a professional employee uses excessive force in the discipline of students or negligence resulting in bodily injury to students.
Tex Educ.Code ANN. § 22.051 (Vernon 1996). This section then establishes an affirmative defense for professional school employees.
The Supreme Court of Texas determined the circumstances to which this defense applies in Barr v. Bernhard. In Barr, the Supreme Court interpreted the predecessor to section 22.051—section 21.912. In addressing an ambiguity in the language of section 21.912, the Court held that section 21.912 meant that a professional school employee is not personally hable
for acts done within the scope of employment, and which involve the exercise of judgment or discretion, except in circumstances where disciplining a student, the employee uses excessive force or his negligence results in bodily injury to the student.
Barr, 562 S.W.2d at 849. The Supreme Court later adopted this interpretation for section 22.051. See Hopkins v. Spring Independent School District, 736 S.W.2d 617 (Tex.1987). Under this interpretation, the defendant-employees must prove the following elements to establish an affirmative defense under section 22.051:
(1) they were professional employees of the applicable school district at the relevant time;
(2) them actions, if any, were incident to or within the scope of their duties at the relevant time;
(3) their duties involve the exercise of judgment or discretion; and
(4) they did not use excessive force in the discipline of the student.
On appeal, the Fosters argue that the defendants did not conclusively establish these elements. Specifically, the Fosters challenge the defendants’ evidence as it relates to the third element. Thus, we must examine the summary judgment evidence to determine if Estrada and Gonzalez conclusively proved that “their duties involve[d] the exercise of judgment or discretion.” Tex. Eduo.Code Ann. § 22.051 (Vernon 1996).
Evidence Supporting the Defense
To support their defense under section 22.051, Estrada and Gonzalez submitted affidavits in which each asserted that he was a professional employee of the Alice School District, that he had acted within the scope of his employment, that his actions involved the exercise of judgment and discretion, and that he did not use excessive force to discipline William, Jr. These affidavits were the only evidence that Estrada and Gonzalez presented to support their motion. In their first issue, the Fosters contend that this evidence was insufficient to prove that the defendants’ omissions were discretionary as a matter of law. Specifically, the Fosters argue that the affidavits do not prove that defendants’ omissions—failing to prevent William, Jr. from entering the gymnasium—involved the exercise of judgment and discretion. We agree.
In the affidavits, Estrada and Gonzalez each stated that “my actions involved the exercise of judgment and discretion on my part.” By making this statement, the affi-ants merely asserted a legal conclusion. The affidavits do not demonstrate how or why the disputed omission is legally characterized as involving the exercise of judgment or discretion. Instead, the statements are selfserving assertions. Because each statement merely states a legal conclusion, the affidavits are insufficient to support summary judgment as a matter of law.2 As a result, Estrada and *754Gonzalez failed to met them burden in establishing a defense under section 22.051.
Even if the affidavits were sufficient to prove an affirmative defense, the evidence submitted by the Fosters raised a fact question about an essential element of the affirmative defense. In response to the motion for summary judgment, the Fosters submitted an excerpt from the Williams Adams Junior High School Faculty Handbook. This excerpt described “Morning Duty for Teachers” for the time period “7:30 a.m. to 7:50 a.m.” and used a map to assign responsibilities to teachers for “ensuring] a safe and orderly environment.” The map designated the gymnasium as Area # 1. The description for Area # 1, reads: “Direct students toward campus and do not allow students to walk to Bryson’s and/or leave campus.” The main school building was designated as Area # 7. The description for Area # 7 reads: “Allow only those students that are in for tutoring inside the building.” The Fosters relied on this evidence to contend that faculty members were under a mandate to “prevent students from entering school building(s) until 7:50 a.m.” and that this act was ministerial in nature. As a result, the Fosters argued, failing to prevent William, Jr. from entering the gym — the omission in dispute — did not involve the exercise of judgment or discretion. Although this evidence does not conclusively establish that the omission in dispute was ministerial in nature, it does raise a fact question about whether the omission was properly characterized as involving judgment and discretion. As a result, we sustain the Fosters’ first issue.
Because our disposition of the first issue controls our disposition of this appeal, we will not specifically address the Fosters’ remaining issues. We reverse the trial court’s order granting summary judgment in favor of Estrada and Gonzalez, and remand the case for further proceedings in accordance with this opinion.
Dissenting opinion by GREEN, J.
. A "professional employee" includes:
(1) a superintendent, principal, teacher, supervisor, social worker, counselor, nurse, and teacher’s aide;....
(4) any other person whose employment requires certification and the exercise of discretion.
Tex. Educ.Code Ann. § 22.051 (Vernon 1996).
. See Anderson v. Snider, 808 S.W.2d 54, 55 (Tex.1991) (reversing because trial court granted summary judgment based on affidavit in which defendant merely denied negligence or malpractice); Mercer v. Daoran Corp., 676 S.W.2d 580, 583 (Tex.1984) (determining that affiant's assertion that disputed indebtedness had been renewed and extended was insufficient to establish existence of a fact in support of motion for summary judgment); Brownlee v. Brownlee, 665 S.W.2d 111, 112 (Tex.1984) (reversing in part because, in establishing his affirmative defense, defendant’s affidavit asserted a legal conclusion that his contractural obligation to pay child support and alimony had been modified); Davis v. City of Palestine, No. 12-95-00180-CV, 1997 WL 530747 (Tex.App.—Tyler Aug. 27, 1997, no writ), at *5-6 (reversing summary judgment because *754affiant's assertion that the defendants had carried out their duties in good faith and in conjunction with other city officials in determining how the needs of the city could be met did not establish the defendants’ entitlement to official immunity); Gallegos v. Escalon, 918 S.W.2d 62, 64 (Tex.App.—Corpus Christi 1996, no writ) (reversing because former school superintendent’s affidavit asserting that providing information to members of the board of trustees in response to questions was part of his duties did not conclusively prove that his statements were made within the scope of his employment); Newman v. Obersteller, 915 S.W.2d 198, 201-02 (Tex.App.—Corpus Christi 1996, writ granted) (determining that school superintendent's affidavit stating that school coach was given wide latitude in performing his job contained legal conclusions and was insufficient to establish affirmative defense provided in Education Code), rev 'd. on other grounds, 960 S.W.2d 621 (1997).