Uh... No matter the religious views, there are articles of clothing which are prohibited in school. Even normally protected expression can be limited in a school setting if it causes a disruption. That's been upheld over and over again by the Supreme Court(Tinker v. Des Moines, Bethel School District v. Fraser, and Hazelwood v. Kuhlmeier).
Religious or not, if it caused a disruption in a classroom, or reasonably could be expected to, the school has the right (in loco parentis) to step in and make him change. That's how the school system functions.
The funny part, of course, is that all the people saying "it's an awesome religion, I hope the principal gets punished" would sing a different tune if they didn't just think the religion was kookie and fun. If, for instance, his religion required him to dress in clothes emblazoned with racial and ethnic slurs, no one would be stepping up and saying "b... But it's his religion".
Either be in favor of all disruptive religious expression in school, or none of it