No one sees anything wrong with that?
This is a good life lesson for the kid with the NRA shirt.
First, The establishment enforces the rules, be it the state, federal gov't or just a po-dunk local school district as they see fit and when it suits them. They also make them up on the fly when needed.
Second, just because you are not violating any laws/rules does not mean you cannot be arrested.
The kid did the right thing in not resisting the arrest or he would be facing some real difficult charges. Sometimes its just best to call the lawyers.