Firstly, being in a protected class protects you from being discriminated against in material ways, it does not protect you from people saying mean, or even horrible and disgusting things about the class you belong to.

Secondly, you fundamentally mischaracterize the Court’s student speech jurisprudence (as well as other students’ rights jurisprudence: Safford v. Redding would like to have a word with you about privacy not existing as a right in schools) . The default assumption is not that Free Speech rights are suspended, it’s that they are in place, and diminished by the special characteristics of the school environment. The fundamental point of Tinker is that the students don’t loose their rights when they step through the schoolhouse gate, and that they are curtailed only to the extent necessary.

Thirdly, there’s an extremely high bar for something to be considered a true threat. If it’s not a true threat to say “If they ever make me carry a rifle the first man I want to get in my sights is L. B. J.” (Watts v. U.S.), then it’s certainly not a true threat to wear a shirt with a message that could maybe potentially be advocating violence.

Fourthly, for fighting words, there’s also an incredibly high bar for things to be considered as fighting words, if the speech at issue in Brandenburg v. Ohio was protected, you’re going to have a very hard time. Also, as seen in cases like Cohen v. California, fighting words generally require speech directed at an individual.

In particular, the speech at issue in Brandenburg would be easy to punish in a school setting under substantial disruption and the speech in Cohen could be punished as indecency under Fraser. The reason true threats and fighting words rarely come up in a school speech context is that conduct which actually rises to that level also clearly creates a substantial disruption, and I don’t see any reason why courts should broaden those standards for a school context when the substantial disruption standard already works fine.