Going after someone who's seventeen and consenting is sleazy, and shows an almost complete lack of judgment, but it's not quite the same thing as molesting a child, or forcibly raping someone. The legal problem, as I understand it, is that the principal was in a position of trust, not that the student was too young -- a quick search suggests that she wasn't.

I suppose I see most of the problem here coming from pure stupidity. The principal knew what was required of him in his job, should have been aware of the law, should have known how to conduct himself -- and ended up behaving like one of his students. Definitely not someone who deserves to be teaching, but I reserve fear and indignation for those who use force and coercion, and who stalk literal children. This guy just gets a whole lot of disdain.

Am editing this because I don't want it to seem creepy. I agree that what happened was absolutely wrong, and don't want to sound like I'm defending this guy, I just think there are worse crimes out there. I've also been in high school recently enough to know that a lot of the students are up to things they shouldn't be, but that doesn't mean it's okay for a figure of authority to involve himself too!