In all cases where, prior to the time this Act becomes effective, an election has been duly called and held by the governing body of any school district in manner and form provided by law, for the purpose of submitting the proposition of increasing the annual educational tax rate of said school district, and the proposition of issuing bonds of said school district for school building purposes, and a majority of the legal voters voting on each of such propositions voted in favor thereof, and the face of the ballots used to vote upon each of said propositions, respectively, was in the form provided by statute and each ballot stated the proper purpose, and there appeared on the back of each such ballot the election precinct and polling place at which such ballot was to be used, each such ballot voted at such election be and is hereby declared to be legal and valid notwithstanding that the ballots used at such election contained an erroneous statement of the proposition submitted on the reverse side thereof.