That said, this is one of the most stupid things I have ever seen:
1) If the school has a policy of not wearing political t-shirts then she can't. If there is no such policy, then she can, but has to be prepared for others to use their First Amendment rights to peacefully tell her that they don't agree.
2) The very fact that she is not in prison (or worse) shows that she is not having her First Amendment rights violated. Her opinion has been expressed and she has gotten much more coverage than my opinion ever has.