I'd have gone ballistic. I come from a long line of people who are quite expert at making points forcefully when the circumstances warrant it.
Years ago, my brother had a bully issue (he was the victim) and the school wasn't doing anything about it though the bully was well known to the adminstration. My mother grabbed a baseball bat and went to see the principal of the school, bat in hand, and ever so gently advised the principal that she was an Air Force wife with a husband currently on a year-long unaccompanied tour in Alaska, and she would do whatever it took to ensure that her children could walk to and from school and not be molested by a bully, or during school as well.
The principal got quite nervous, understandably. But the problem went away.
Being from that genetic line, in your situation I'd have become VERY loud with these bastards but not to the point where I was in any danger of getting arrested. I'd have also told them that their anti-gun agenda qualifies them as shitheads who don't understand or believe in the rights that are guaranteed in this country. I would have also ORDERED them to take a 'hands off' approach toward my son. If he wants to read a hunting/sportsman/outdoors magazine or any other non-pornographic material in his spare time, even during class, it's his right to do so and they're not to interfere. Hell, I'd buy him a subscription to Small Arms Review and Soldier of Fortune and let him carry them to school after that!
My attitude, stated simply is: I'm the father. I teach him how to be a citizen. The school is to teach him reading, writing, math, science, physical education, history, and a few other specific areas of knowledge, but are NOT permitted to push their liberal views on my kid and they are most CERTAINLY not allowed to be the ones to administer discipline. That's MY job!
If my kid was given detention, my standing orders would be that he is to ignore the detention order and would come home at the normal time, and I would handle it. (not unkindly, by the way.) If the school didn't like it, tough shit. I can take him to a private school if I want.
The job of the school is to educate. Nothing more. Information only, and NOT including political indoctrination.
The job of the parent is to turn their children
into decent citizens. That includes the responsibility for all discipline.
CJ