Missouri Revised Statutes Title XI. Education and Libraries § 160.041. School day, school month, school year, defined -- reduction of required number of hours and days, when

1. A “school month” consists of four weeks of five days each for schools with a five-day school week or four weeks of four days each for schools with a four-day school week. The term “school day” shall mean any day in which, for any amount of time, pupils are under the guidance and direction of teachers in the teaching process. The “school year” commences on the first day of July and ends on the thirtieth day of June following.

2. Notwithstanding the provisions of subsection 1 of this section, the commissioner of education is authorized to reduce the required number of hours or days in which the pupils are under the guidance and direction of teachers in the teaching process if:

Such reduction shall not extend beyond two calendar years in duration.