The sums credited by the Board to each school district, under the provisions of §§ 59-21-340 and 59-71-550, shall remain available to school districts until requisitioned by them for purposes approved by the Board. Such funds shall be available for the following purposes only and in the following order of priority: (a) To be applied on the financing of capital improvements approved by the Board; (b) to pay principal and interest of school district indebtedness represented by bonds or notes issued before July 1, 1951 for any capital improvement or bonds or notes issued on or after July 1, 1951 for capital improvements approved by the Board.