Yep... agenda
Congrats Mom and Dad... you dunn well...
I am reminded of that guy in California who told his daughter to refuse to read the Pledge of Allegience because it contained "under God," so he would have standing to bring a lawsuit to challenge the Constitutionality of it. To me this is a form of child abuse.
In the present case it does appear this kid agrees with his father about the NRA, but he is 13 years old and honestly I don't care a great deal what he thinks. He and his parents are certainly forcing the school's hand by continuing to wear the shirt.
I think, were I the principal, I would probably convene some kind of assembly to discuss this in an orderly way, and articulate a) why the school initially thought the shirt was unacceptable and told him to turn it inside out; b) why the school will not tolerate students creating disturbances; and c) the fact that the school will allow kids to wear the shirt so long as they are not being disruptive. I expect the whole thing would blow over in a few weeks. Of course, the fact that the family has now retained a lawyer complicates things somewhat, but that would not prevent me handling it more or less this way.