Sale or exchange of bonds – Use of proceeds. Refunding bonds issued under Section 34-65 may be exchanged on the basis of par for par for the bonds being refunded and described in the authorizing resolution, or may be sold at not less than par under the direction of the board, and the proceeds thereof shall be received by the city treasurer, as school treasurer, and shall be kept in a separate fund to be used solely for the purpose of paying the principal and interest on the bonds so refunded. All bonds refunded shall be cancelled.