Hmph, no NTR or cheating tag. It looked like they took the relationship seriously even as illicit as the student-teacher relationship is. The vice principle hijacked it by compromising her devotion to her lover so I can't see why this isn't netorare.
Ah yes, the moment you see that old man eyeing her with a knowing gaze, it was pretty obvious what would happen.
How tragic, though. I feel if she had just told him about it, he would have given everything up just to be with her, because he's a nice guy that said he'd always be on her side. He definitely would not be happy if he found out what she was doing, even if it's for his sake.
It's sad, really. It probably wouldn't realistically be easy to work out a situation like theirs, but resorting to being NTR'd for the sake of protecting him is sure to leave a permanent scar on the relationship, especially if he does eventually find out (most likely in the worst way possible, witnessing it happen).
If you are going to fuck your students, keep that shit to the next town over until they graduate. At least use a No Tell Motel and enter and leave at different times.