You need to chose your battles. You sound like you're willing to mess with your own life over something someone did in their life. Did this teacher do anything to you? Is it personally affecting you or any of your classmates, other than it taking up head space? I'm willing to bet that something else is up and this is a front. Are there more factors stressing you out than just discovering this teacher's record? I've wanted to remove myself from situations before and pointed to reasons that weren't the true reason. Even if it is your reason, it's still a you problem. The school sees it fit to keep this teacher on staff, so whatever they did probably doesn't bar them from employment so it's just something that you need to work through your problem with. Like other people said, just transfer out of the class.