The rule needs to bear some semblance to necessity, and not just satisfy the stylistic aesthetic whims of a geezer.
If the kid performs, and it does not impede anyone else, then his hair is not business of any teacher. Im for the kid. So no, the true “liberal viewpoint” is the impulse to tell other people how they should live even though it does not affect you.
How about a rule that he must part it on the right?
This was an extracurricular activity. Play by the rules or don’t play.