Wouldn't it count as freedom of expression?
https://www.liberty-human-rights.or...uman-rights-act/article-10-freedom-expression
There are many groups who would consider body art a valid media of artistic expression and I don't see anything that denies minors this protection (I see some exceptions, such as the right to bring a court case but nothing sweeping).
Also minors have additional rights (a child's right not to be denied education), which haven't been violated yet, as he is isolated and not excluded:
http://www.theguardian.com/uk/2006/feb/06/schools.religion
Schools really should be more careful when drawing up regulations or imposing punishments, it most certainly isn't like when we were growing up.