Bonds authorized by the provisions of this act¹ shall be issued and sold at not less than par, and shall bear interest at a rate not exceeding four per cent (4%) per annum, payable in not more than twenty-five (25) years from the date of said bonds. Said bonds shall be serial bonds and shall mature in annual installments, as nearly equal as the bond issue will permit, beginning not later than five (5) years after the date of the bonds: Provided, that the above provisions shall constitute the minimum requirements for the maturity of bonds and nothing herein shall be construed to prevent a school district of the first class from providing in any bonds issued under this act for accelerating such maturities.