The school district shall provide on an annual basis, while operating a school on a four-day school week, not less than 560 hours of instructional time for kindergarten, not less than 700 hours of instructional time for grades 1, 2, and 3, and not less than 845 hours of instructional time for grades 4 to 12, inclusive. The school district may exceed these minimum instructional times except that no pupil in a kindergarten shall be kept in school more than four hours in any day, exclusive of recesses. The school day may not exceed eight hours, nor may the school week be less than four days.