Quite right. As a teacher, albeit a trainee one, you are supposed to be setting an example to the kids, so green hair would hardly be appropriate. No, it is not a breach of your "human rights" for the school to insist you dress in a sensible way, including your hair. Being held prisoner illegally is a breach of human rights. Being denied an education is a breach of human rights. Being told you can't have green hair while responsible for a class of schoolkids is perfectly reasonable.
Not being rude, but if people spent as much time worrying about their responsibilities as they spend worrying about their "rights", the country would be all the better for it.
Good grief, now you've got me sounding like a Daily Mail reader. Yuck.