At the time of issuing said bonds the regents shall establish for the payment of the principal and interest thereof a fund to be known as “interest and retirement fund” into which fund said regents 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 (12) months, and annually thereafter shall continue to place in said fund a sufficient amount to pay principal and interest maturing in the succeeding twelve (12) months.