Many school policies do prohibit images of weapons on clothing, but without that explicit prohibition in writing, the school is legally in the wrong. I'll bet they will have a written policy on that by the start of the next school year if not sooner. As the father, I would (1) sue, as much as I despise most lawsuits, and (2) buy a wardrobe with a broad range of messages to push the buttons of the liberal whiners in that school: Second Amendment quotes, NRA quotes, and everything pro-freedom but without images that violate the letter of the dress code.
There are so many options, many of which are particularly fun or educational. If I stayed in that school, I'd do it - and warn the disgustingly wimpy teacher not to retaliate in grades or in any other manner unless she wants to face years of lawsuits. Classmates should learn where Molon Labe comes from - it adds to the coolness of independence and of the ability to defend yourself instead of waiting for the police to come with your toe tag. Classmates should learn the history and modern meaning of the Gadsden flag - again high in coolness.