I was a teacher a decade ago. It was during a time when nationally, school violence was an issue. I'm a huge "concealed carry permit" guy. And I was never able to carry on campus, even though I worked at one of the toughest schools in this part of the state. Some of my students had arrests for serious crimes, and violence was sometimes their first option. They talked about weapons all of the time, as it was middle school and they're little grownups at that point. When a poor, angry kid from a violent home starts puberty, they can really be unpredictable. Put 30+ of them in a small classroom, it can be a powder keg.
All you can do as a teacher is smile, pray for the best, and prepare every way you can, inside the system. I always placed my steel desk at the best strategic angle to the door. I kept a spare flagpole and more importantly, a chunk of cinder block (used by the previous teacher as a door jam) handy, to use in case I had to protect this room full of students from a shooter on campus. And if I ever heard shots on campus, I would ORDER every student in that class to ATTACK that shooter the moment they entered - and I'd be leading that charge! I might catch a bullet in the belly, but the group will take that shooter down.
it wasn't perfect, not by any measure. BUT if every teacher had this kind of aggressive plan - where the shooter KNEW he woudl be getting his head beat in, even if he took a few people down first - then people might not try such madness. I admire the school workers that took down that recent shooter earlier this year. Would love to see more of that - these are the biggest heroes of our society.