Each day within the school term is a day in session except Saturdays, Sundays, and days designated as holidays by or according to AS 14.03.050. A school board may approve Saturdays as a day in session. The day in session in every school shall be at least four hours long, exclusive of intermissions, for the first, second, and third grades and five hours, exclusive of intermissions, for all other grades. The commissioner may approve a shorter day in session for any grade. The period of the day in session shall be devoted to the instruction of pupils or to study periods for the pupils.