Moral Dilemma: What would you do if...

No, it's unprofessional and unethical for a teacher to form a sexual and romantic relationship with a student. Waiting until the student is 18 means the teacher has already acquiesced to an unacceptable connection.

If the teacher is so hard-up that this underage girl is his only option, then he should quit his job and wait for her to turn 18.

But nah, he has to choose his job or the girl. Or better yet, go find a relationship with someone your own age whom you have no professional power over.

The teacher is a f*****g loser.