They have dress codes and they expect a certain standard from you.
only if it's written in your contract, if not then it's their own fault.
In the schools I've worked in I've never had problems and I've piercings and tattoos and have never even once had to wear a suit, not even proper trousers, just jeans, i thought I'd have had problems when it got warm in the summer with shorts but nah, even one else wears them.
If your boss has a problem with your hair colour then he's a tossbag and not worth working for