E. Indebtedness contracted for school purposes, evidenced by bonds, to the extent to which state aid for common schools, not exceeding two million five hundred thousand dollars, shall meet the interest and the annual requirements for the amortization and payment of part or all of one or more issues of such bonds. Such exclusion shall be effective only during a fiscal year of the city in which its expense budget provides for the payment of such debt service from such state aid. The legislature shall prescribe by law the manner by which the amount of any such exclusion shall be determined and such indebtedness shall not be excluded hereunder except in accordance with the determination so prescribed. Such law may provide that any such determination shall be conclusive if made or approved by the state comptroller.