That the governing board issuing said bonds shall, at the time of issuing said bonds, establish for the payment of the principal and interest thereof a fund to be known as “interest and retirement fund” into which fund said board shall immediately place a sum not less than the amount necessary to pay the interest and maturing principal of said bonds for the ensuing twelve months, and annually thereafter shall continue to place in said fund a sufficient amount to pay principal and interest maturing in the succeeding twelve months.