Unfortunately for the kid and his parents, if this took place in the US, the kids leave their First Amendment Rights at the front door of the school when they enter it. There's been something of a legal tradition upholding that minors do not have the full right to Freedom of Speech/Expression, and this specifically holds true in many cases when it comes to school. Sad but true. It could be the world's best question, but if the school thinks it's inappropriate there's not much you can do but appeal.