During the period of time given the board to levy a tax on property within the district for incidental expenses as that term is used in sections 247.010 to 247.220, 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 incidental tax levy, and to be paid out of such current revenue when accrued. 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 of directors shall execute the bonds on behalf of the district and which shall be attested by the clerk with the seal of the district.