We granted discretionary review to defendants Greg and Karen Trucke to consider their challenges to Iowa’s compulsory education law, section 299.1 (1985). Each defendant has been charged and convicted on two counts of the simple misdemeanor offense of failure to cause their children to attend public school or, in the alternative, to obtain equivalent instruction by a certified teacher. See §§ 299.1, 299.6. Although defendants challenge the enforcement of section 299.1 on the ground that it violates their rights under the first and fourteenth amendments to the United States Constitution, we recognize a duty to avoid constitutional questions when the merits of a case may be fairly decided without facing them. In Interest of J.A.N., 346 N.W.2d 495, 498 (Iowa 1984). Because it is apparent from the face of the complaints filed against the defendants that the State not only failed to charge an offense under section 299.1, but at the time the prosecution commenced no crime had yet been committed, the judgment and sentence of the district court based on those complaints must be reversed.1
Section 299.1 requires the parent of a child aged seven to sixteen to cause that child “to attend some public school for at least one hundred twenty days in each school year, commencing with the first week of school after the first day of September.” The statute goes on to provide that “[i]n lieu of such attendance such child may attend upon equivalent instruction by a certified teacher elsewhere.” Violation of the statute constitutes a simple misdemeanor. § 299.6. Because the statute is penal in nature, we construe its terms strictly against the State. State v. Koplin, 402 N.W.2d 423, 425 (Iowa 1987); State v. Soppe, 374 N.W.2d 649, 652 (Iowa 1985).
The record in this case reveals that during the 1984-85 school year defendants’ children, Shawn and Wendy, attended public school. However, beginning with the fall 1985 school term, Karen Trucke began teaching her children at home with the assistance of a certified teacher, Susan Goodenow, who instructed the children approximately four hours per week.
After correspondence between the school superintendent and the Truckes failed to satisfy the school district that defendants were in compliance with the equivalent instruction exception to section 299.1, criminal prosecution was commenced. Complaints filed October 1, 1985, charged that “on the 30th day of September, 1985” Greg and Karen Trucke violated sections 299.1 *244and 299.6 with respect to the schooling of Shawn and Wendy.
Trial by magistrate resulted in conviction on all four counts. On appeal to the district court, the convictions were upheld. In its opinion, the district court held that the State proved the following facts beyond a reasonable doubt:
1. The defendants are the parents of two children, Shawn Trucke and Wendy Trucke, and the defendants have control of their children.
2. Shawn Trucke and Wendy Trucke are over seven and under sixteen years of age.
3. Shawn Trucke and Wendy Trucke are in proper physical and mental condition to attend school.
4. Shawn Trucke and Wendy Trucke did not attend any public school on September 30, 1985, or on any prior day during the 1985-1986 school year.
5. On September 30, 1985, and on all prior days during the 1985-1986 school year Shawn Trucke and Wendy Trucke received no instruction other than instruction received in a home school.
6. The defendants were the teachers who provided most of the instruction in the home school.
7. Neither of the defendants is a certified teacher.
Conceding that the term “equivalent instruction” may be unconstitutionally vague,2 the district court premised its ultimate finding of guilt “on the defendants’ failure to have their children attend a school which provides instruction by a certified teacher for at least one hundred twenty days during the school year.”
We have no quarrel with the facts found by the district court, and we accept without deciding the correctness of its interpretation that the statutory alternative requires 120 days of instruction by a certified teacher. However, we disagree with the court’s prospective application of the statute to these defendants based on complaints which, on their faces, fail to adequately charge an offense under section 299.1.
Contrary to the meaning given the statute by the district court, the statute does not require parents to send their children to a school which provides 120 days of acceptable instruction. By its clear language, the statute requires parents to send their children to public school “for at least one hundred twenty days in each school year ... commencing the first day of September,” or the parents must provide some legally acceptable alternative. Even if we assume arguendo that the statutory alternative also requires 120 days instruction, no parent — including the Truckes — could be in compliance with the statute by September 30 of any given school year, inasmuch as only thirty days of the school year has passed. Excluding weekends and holidays, the Truckes still had approximately 220 days left in the year to comply with the statute.
Hypothetically, a case might arise in which the State could prove the impossibility of meeting the 120-day requirement given the remaining days of the school year. No such attempt was made here. Instead, the record before us on appeal reveals that an essential element of the offense (120 days school attendance) was completely omitted from the charging instrument and the criminal culpability of the defendants was thus determined upon what amounts to no more than their anticipated violation of the statute.
In Johnson v. Charles City we recognized that education is “perhaps the most important function of state and local government.” 368 N.W.2d at 74 (citing *245Brown v. Bd. of Educ., 347 U.S. 483, 493, 74 S.Ct. 686, 691, 98 L.Ed. 873, 880 (1954). But where criminal sanctions are invoked to further this noble purpose, the State cannot dispense with adherence to fundamental rules of criminal procedure. Where, as here, elements such as time and the means by which an offense is committed are material ingredients of the offense, the charging document must so state. Iowa R.Crim.P. 4(7)(c) and (d); 5(5); 33; 36(3); United States v. Manuszak, 234 F.2d at 423; United States v. Purvis, 580 F.2d at 858. The complaints here do not meet that standard and erroneously led to convictions based on crimes not yet committed.
The case is remanded to the district court for entry of an order dismissing the prosecutions.
REVERSED AND REMANDED.
All Justices concur except HARRIS J., and REYNOLDSON, C.J. and McGIVERIN, J., who dissent.. Defendants’ failure to preserve error on this ground for reversal has not been waived. Iowa Rule of Criminal Procedure 10(2)(b) provides that defenses or objections based on defects in the information must be raised before trial unless the information "fails ... to charge an offense ... which objection shall be noticed by the court at any time_” Federal courts interpreting the federal counterpart to rule 10(2)(b) have held that, like lack of subject matter jurisdiction, failure to charge an offense may be raised sua sponte by the court at any time. See Fed.R.Crim.P. 12(b)(2); United States v. Purvis, 580 F.2d 853, 858 (5th Cir.1978), cert. denied, 440 U.S. 914, 99 S.Ct. 1229, 59 L.Ed.2d 463 (1979); United States v. Manuszak, 234 F.2d 421, 423 (3d Cir.1956).
. In Johnson v. Charles City Community Schools Board, 368 N.W.2d 74, 80 (Iowa 1985), cert. denied, 474 U.S. 1033, 106 S.Ct. 594, 88 L.Ed.2d 574 (1985) we alluded to such a challenge, observing that “[w]hen the state fails to adequately set necessary minimum standards for private religious schools, the patrons are presented with a ready defense to truancy charges.” The United States Court of Appeals for the Eighth Circuit recently confirmed our view, holding the undefined "equivalent instruction" requirement unconstitutionally vague. Fellowship Baptist v. Benton, 815 F.2d 485, 495-96 (8th Cir.1987). As noted by the court of appeals, the State has since promulgated regulations entitled “Equivalent Instructions Standards” which became effective February 6, 1986. See 670 Iowa Admin. Code ch. 63.1-4.