Is this a private school? Because that's the way it sounds. If it is or isn't here is my opinion of it.
If they are going to a public school then they have the right to do anything they want to do with their hair. The public school system only has a right to step in IF what they are doing is harming themselves or somebody else. They don't have a right to tell another human being what they can and cannot do.
If they enlisted in a private school and knew what the school expects...then they have no right to get mad about this. They signed up for it. Everybody knows that private schools are more strict. If they don't like the rules...then leave. It's a private school what do they expect...
"The difference between the right word and the almost right word is the difference between lightning and the lightning bug."