My school has uniforms, but I wanted to wear a shirt on the 10th that supported a well-known cause. The principal said no. It's Suicide Prevention (the cause). Isn't it against my first amendment rights that he isn't letting me wear that shirt?
Schools have carved out many special exceptions to facilitae educating the youth of America.
That said, you have very few rights in a school.
Perhaps I should say, you have fewer rights.
At any rate, you can wear the shirt.
They can't prevent you from doing that.
So, you are allowed to express yourself.
But, they reserve the right to sanction, suspend, isolate, or expel you.
You do have "due process" of sorts during the adjudication of any sanction or punishment.
Finally, if you're a minor (as in not an adult) you have no rights.
Your parents have rights they pass to you, because the courts consider minors to be legal incompetents.