Any city or town of the State of Alabama that may hereafter construct or purchase school buildings, a waterworks plant, gas plant, electric light plant, or other light and power plant or extend or enlarge a waterworks plant or light and power plant then owned by such city or town, may, through its mayor and aldermen or other governing body execute a mortgage on the school buildings, the waterworks plant or light and power plant purchased or constructed by such city or town to secure the bonds and indebtedness and interest on such bonds and indebtedness created in the purchase, construction, extension or enlargement of such school buildings, waterworks plant or light and power plant, such mortgage to be signed by the mayor and countersigned by the clerk of said city or town or by such other person or persons as the mayor and aldermen or other governing body of such city or town may direct by appropriate resolution. If, in the judgment of said mayor and aldermen or other governing body of said city or town, it is desirable that such mortgage be executed before such school building or plant is constructed and if it is so executed for the purpose of providing money to construct such building or plant, the fact that such mortgage is executed before the construction of such building or plant shall not render such mortgage invalid.