The principal of and interest on such notes shall not be general obligations of the board of education issuing said notes but shall be payable from the principal proceeds from the sale of the warrants with respect to which such notes are issued and, to the extent necessary, from the tax proceeds which the issuing board of education may pledge to the payment of such warrants. Any board of education may secure payment of the principal of and interest on the notes by a pledge of so much as may be necessary therefor of such tax proceeds.