Whenever any school district in any borough or township of this commonwealth shall have heretofore created an indebtedness for a lawful purpose, by action of the legal and proper officers thereof, such indebtedness being within the constitutional limit of seven per centum and in excess of two per centum of the last assessed valuation of such school district, and not having first obtained the assent of the electors thereof in favor of increasing such indebtedness as provided by law, it shall be lawful for the proper officers of such school district to cause to be submitted to the electors of such school district the question of validating and giving binding force to such indebtedness theretofore attempted to be created.