If I were to bring my AK47 to school. I would bring it in a guitar case. If anyone asks me: what kind of songs can you play? I'll unzip it slightly and say "Death Metal".
Joking about killing school age children is disturbing, is this something you've thought about doing?
It's just a joke to illustrate the mass shooting at schools. You don't have to deny me my fundamental rights because of a bad joke
Your rights aren't being denied, it's just an upsetting topic to joke about. Have you considered talking to any school shooting survivors or the parents or friends of those that lost someone in a school shooting? It may change your perspective.