There shall be a common school fund which shall consist of the poll tax (to be retained in the counties where the same is collected) and an additional sum from the general fund in the State treasury which together shall be sufficient to maintain the common schools for the term of four months in each scholastic year. But any county or separate school district may levy an additional tax to maintain its schools for a longer time than the term of four months. The common school fund shall be distributed among the several counties and separate school districts, in proportion to the number of educable children in each, to be determined from data collected through the office of the state superintendent of education, in the manner to be prescribed by law.