Is the prayer led by a man who follows a school of thought different from that of the People of the Sunnah valid, and is it obligatory to repeat the prayer if it is not valid, and what should one do if they did not know that he was the Imam?

Answer: One should not pray behind an innovatively-inclined Imam if there is another Imam available. However, praying behind him is valid for whoever knows his condition or is unaware of it. The congregation should have put forward a person from among the Sunnis to lead the prayer.