Each elementary, high school, and unified school district shall report to the Superintendent of Public Instruction on forms prepared by the Department of Education in addition to all other attendance data as required, the active enrollment as of the third Wednesday of each school month and the actual attendance on the third Wednesday of each school month; except that if such day is a school holiday, the active enrollment and actual attendance of the first immediate preceding schoolday shall be reported. “Active enrollment” on a day a count is taken means the pupils in enrollment in the regular schooldays of the district on the first day of the school year on which the schools were in session, plus all later enrollees, minus all withdrawals since that day.