When any bonds shall have been issued by any county, city, incorporated town or village, school district, or other political corporation or subdivision of the state, as provided under the constitution and laws of this state for the incurring of indebtedness, or for refunding, extending, unifying the whole or any part of their valid bonded indebtedness, the proceeds from the sale thereof and all moneys derived by tax levy, or otherwise, for interest and sinking fund provided for the payment of such bonds, shall be kept separate and apart from all other funds of such governmental unit, so that there shall be no commingling of such funds with any other funds of such county, city, incorporated town or village, school district, or other political corporation or subdivision of the state; provided, that in no case shall the proceeds derived from the sale of any such bonds be used for any purpose other than that for which such bonds were issued, nor shall such interest and sinking fund be used for any purpose other than to meet the interest and principal of such bonds; provided further, that any bonds or money remaining in the interest and sinking fund of any such county, city, incorporated town or village, school district, or other political corporation or subdivision of the state, after the extinction of the indebtedness for which such bonds were issued, shall be paid into the general revenue fund of such county, city, incorporated town or village, or other political corporation or subdivision, and into the capital projects fund of such school district.