During the period of time required to collect any tax or property within the district levied for general operating purposes the board may issue and sell current revenue bonds to bear interest at not to exceed six percent per annum to meet the current expenses of the district incurred in advance of the revenue to be derived from such tax levy, and to be paid out of such current revenue levied for general operating purposes when collected. Such bonds may be for six, nine or twelve months and shall not exceed in the aggregate one-half of the revenue for the fiscal year for which they are issued. The president of the board shall execute the bonds on behalf of the district and they shall be attested by the secretary with the seal of the district.