This is WRONG. Teachers are not the political gestapo.
If your school allows free dress, including dress that supports political views, than NO TEACHER OR PROF has the right to humiliate  a student wearing one the teacher does not like.
This is America, not Animal Farm.   Granted it was a math teacher, not a history teacher, so an allowance can be made for her not upstanding the Constitution. But she owes that class and that kid an apology before she is allowed to teach again.

http://www.nbcphiladelphia.com/news/politics/Romney-High-School-T-Shirt-Flap-172627871.html