At the time bonds payable in whole or in part from taxes are issued, the board shall levy a continuing direct annual ad valorem tax for each year while all or part of the bonds are outstanding on all taxable property within the district in sufficient amount to pay the interest on the bonds as it becomes due and to create a sinking fund for the payment of the principal of the bonds when due or the redemption price at any earlier required redemption date and to pay the expenses of assessing and collecting the taxes.