Any school district in the State of Nebraska which has heretofore voted and issued, or which shall hereafter vote and issue, bonds to build or furnish a schoolhouse, or for any other purpose, and which bonds, or any part thereof, still remain unpaid, and remain and are a legal liability against such district and are bearing interest, is hereby authorized to issue refunding bonds with which to call and redeem all or any part of such outstanding bonds at or before the maturity or the redemption date thereof, and may include various series and issues of the outstanding bonds in a single issue of refunding bonds, and issue refunding bonds to pay any redemption premium and interest to accrue and become payable on the outstanding bonds being refunded. The refunding bonds may be issued and delivered at any time prior to the date of maturity or the redemption date of the bonds to be refunded that the school district determines to be in the best interest of the district. The proceeds derived from the sale of refunding bonds issued pursuant to this section may be invested in obligations of, or guaranteed by, the United States Government pending the time the proceeds are required for the purposes for which such refunding bonds were issued. To further secure the refunding bonds, the school district may enter into a contract with any bank or trust company, within or without the state, with respect to the safekeeping and application of the proceeds of the refunding bonds and the safekeeping and application of the earnings on the investment. Any outstanding bonds, which shall have been called for redemption and which have sufficient funds or obligations of, or guaranteed by, the United States Government set aside in safekeeping to be applied for the complete payment of such bonds, interest thereon, and redemption premium, if any on the redemption date, shall not be considered as outstanding and unpaid bonds. All bonds issued under the provisions of sections 10-717 to 10-719 must, on their face, contain a clause that the district issuing such bonds shall have the right to redeem such bonds at the expiration of five years from the date of the issuance thereof.