Based on the arguement and logic of the negative connotations of symbols of oppression, I can make a very good case for banning display of the American flag. (Or most national flags, I suspect) and probably a better one than those made against a "Pride" tee shirt, whether straight, gay or bi.
In another thread, I quoted Justice Edwards. In my not so humble opinion it bears repeating: Those who begin coercive elimination of dissent soon find themselves exterminating dissenters. Compulsory unification of opinion achieves only the unanimity of the graveyard.
When Troll told me the "Straight Pride" story, my first question was: Are Gay Pride or WWJD or similar tee-shirts also banned? if so, its a dress policy issue that's supposed to reduce disruptive behavior. I may think such a policy is silly and repressive and doomed to failure but it has the minimal benefit of being oppressive to the entire school population equally. If not, it is coercive and it's ultimate effect is to teach conformity and hypocracy as appropriate character traits.
Not knowing anything about the boy or parents, I can't judge their beliefs. The principals' characterization of them as "homophobic",if true, has a disturbing hint of Cotton Mather about it.
So to protect teen gays and bi's we label another child as a bigot, or at least the product of a bigoted family. Does wonders for his self concept, no doubt.
Especially if the tee shirt was meant not as some kind of anti-gay statement but more of "ain't I cute and in your face" sort of thing.
The message sent to children is that they aren't competent or mature enough to handle any disturbing influences in their environment and any such must be ruthlessly eliminated. That the answer to such is not to learn ways to handle dissenting opinions, but to mandate that they never be expressed. Sweeping issues under the rug strikes me as inherently dangerous and an admission of failure of self proclaimed adults as it relates to the need and duty to teach concepts such as self reeliance and moral and ethical autonomy to children.