You’re right. Your husband is playing with fire. It’s late for a “mid-life” crisis but such crises of identity are not limited to middle age. Your husband may be feeling old. The attention and admiration of a young woman is intoxicating stuff. She admires him. He bends and breaks rules to show her that she is special and that he can be her hero. She has nothing to lose. But he has everything to lose.
You can’t reason someone out of this kind of thinking. He is a man obsessed. I suggest you try a different tactic. Instead of fighting with him about the facts, try to have a conversation about the real issues. Sympathize with him about how hard it is to feel old. Since he is almost 10 years older than you are, my guess is that he is feeling less attractive and more aware of the passage of time. Let him know that you do find him attractive and that you love him. Ask if there is anything you can do to help him feel better. Tell him how sad it makes you that he is in such a panic about getting old that he is risking everything for a young woman’s approval.
Whenever he starts talking about his obsession with the young teacher, gently remind him that his relationship with her can’t last but that you are ready to be there. Then leave the room. Hopefully he will come to his senses before he finds himself facing disciplinary action at his job.
I wish you well.