Beginning with the school year 1965-1966 and in each school year thereafter, to districts eligible under the density factor, payments on leases or sinking fund charges shall be no less than fifty percent (50%) of the approved reimbursable rental or sinking fund charge for a school building project. Such payments shall be made annually, semi-annually or as may otherwise be required by the terms of any agreement entered into by the school district with the approval of the Superintendent of Public Instruction.¹