Unless otherwise agreed upon, each school district shall pay a proportionate share of the salary and expenses of the superintendent and the expenses of the supervisory union based on the number of enrolled students in each member school district. "Enrolled students" shall be defined by the State Board by rule, including the treatment of tuition students, special education students, students enrolled in career technical centers, and other particular circumstances. (Amended 1961, No. 123; 1987, No. 228 (Adj. Sess.), § 17, eff. July 1, 1989; 1991, No. 181 (Adj. Sess.), § 11; 1991, No. 204 (Adj. Sess.), § 7; 2013, No. 92 (Adj. Sess.), § 43, eff. Feb. 14, 2014.)