The board, with the advice of the Superintendent of Public Instruction, may determine the eligibility of county superintendents of schools to lease portable classrooms provided that a county superintendent of schools is eligible to receive one portable classroom pursuant to this section and Section 17717.2 for each 15 units of average daily attendance at county community schools in excess of the amount of average daily attendance claimed by the county superintendent of schools in the prior fiscal year except that, for pupils who are enrolled in a county community

school and on independent study, only time spent in the classroom shall be included in the calculation of average daily attendance.