Whenever any board of school directors shall have heretofore sold any unused and unnecessary lands, the sale of which is authorized under provisions of the school laws of the Commonwealth, and such board of school directors has received a fair purchase price, then such sale shall be valid and binding on the school district and all deeds of conveyance given by the school district for any such lands are hereby ratified, confirmed and validated, and such purchasers and their respective heirs, successors and assigns shall hold and may convey such titles and estates indefeasibly as to any rights of the school district therein, notwithstanding the fact that the same was not sold by public auction, as provided by law, or upon sealed bids requested by the school board, or upon a private sale subject to the approval of the court of common pleas as provided by law: Provided, That such ratification and confirmation shall not relieve the board of school directors in the event of any fraud or improper act.