The school board of any school district in this state in order to carry out the provisions of sections 178.370 to 178.400, may set aside from its funds for school purposes any sum that is not needed to maintain the elementary and high schools in the district, for the purpose of paying, or contributing to the payment of, the tuition of its students who have completed the high school course and who attend a school that maintains such college courses in any other district. In the event that the sum is not sufficient to pay the tuition fixed by the board the sum shall be prorated among the students taking the college courses. No district shall set aside for the purpose a sum greater than the amount necessary to pay the tuition so fixed by the receiving school district.