The board of education of a Class V school district shall provide for the interest on all existing bonds issued by the district before the interest becomes due. The board shall also, immediately after the expiration of one-half of the time for which such bonds are issued, proceed to set apart each year, for a sinking fund, a requisite amount or proportion sufficient to pay the principal of the bonds when they become due. All money set apart for the sinking fund shall be invested as follows:
(1) In the purchase of and redemption of bonds of the school district, which bonds shall be purchased in the open market in such manner as the board of education prescribes;
(2) In bonds of the city constituting the school district;
(3) In bonds of the county in which such district is situated;
(4) In bonds of the State of Nebraska; and
(5) In United States bonds.