That's a good point. The wife was making a lot of excuses for him, and probably wasn't the most reliable narrator of events. You wonder if maybe she had witnessed interactions herself with students that subconsciously worried her, so that it's not so "clear-cut" as she describes. And, she does shift the blame more onto the teenage girls, which could further blind her to anything her husband is actively doing. In fact there could be plenty of other stuff already going on, or things that happened between him and the 21-year-old when she was younger.
Red flags all around from a relationship point of view! But I wonder if she is correct, that the school won't take any further action since the affair was with another adult. Nowadays I think they would be erring on the side of caution, or at least doing something to further monitor him, when she implies no action will be taken. I wonder if her husband just told her he told his boss about the affair, but he really didn't. So if she wrote a letter to the school board, etc., it might all come as a surprise to them. But I'm curious what they would do--it'd be so easy for the husband to say, "Yeah, that's just my crazy soon-to-be-ex, making up lies and trying to get me in trouble at work." I hope she has some concrete proof, or that the other woman is willing and able to step forward also!