Principal can’t set aside the first amendment on a whim either. The shirt stated a perfectly civil political position.
Principal, and others in that school, don’t have the right not to be offended by civil political discourse. Both the Constitution, despite its many court-induced distortions, and the case law indicate that political speech is still protected speech everywhere (except of course for the McCain/Feingold dreck). It doesn’t matter if you agree with it, so long as its civil - meaning that it doesn’t offend the sensibilities (silkscreen of an aborted fetus on the shirt would offend the sensibilities, for example).
There’s NO suggestion or hint in the First Amendment that the free speech parts of it apply in a classroom — sorry. Go read it sometime.