The county superintendent of education of a county which does not alone or in conjunction with another county maintain an agricultural high school or an agricultural high school-junior college, may provide, with the approval of the county board of education and the board of supervisors, for the attendance of pupils residing in the county of which he is superintendent of education, at an agricultural high school or an agricultural high school-junior college located in a county adjoining thereto, and pay by certificate drawn by him on the county school funds for the instruction of such pupils. However, the amount so paid shall not be greater than the pro rata share of such pupils, in the actual cost of incidentals and tuition, of such agricultural high school or agricultural high school-junior college. Counties co-operating under provisions of this section shall be represented on the boards of trustees by the county superintendent of education.