The projects for which bonds may be issued under this chapter and the periods of maturity thereof, shall be as follows:
When bonds are issued for more than one of such projects, they shall mature within such a period not exceeding the average of the different periods hereinbefore assigned to the several projects for which they are to be issued, as shall be determined by the governing body of the municipality, by which the bonds are to be issued or the board of education of the district, by taking into consideration the amount of bonds to be issued on account of each, and such determination shall be conclusive in any action or proceeding involving the validity of the bonds.
N.J.S. § 18A:24-5