I teach in the inner city and have for the last nine years. I've got gangbangers, murderers, armed robbers, rapists and a BUNCH of really good, bright kids just trying to get through and succeed in life. Two years ago, I moved into my current school that is effectively in the heart of the inner city--a school with a pretty bad rep. The reality is, kids are the same everywhere. In my other school, I was challenged just once. I handled it quickly and effectively and thereafter I had no problems. When I moved (there was a lot of tears from my students at that time middle of the year) a lot of my kids called their buddies at the new school and I never had a problem here either.
BUT....the real problem is not IN school...it's outside the school property. A kid that goes nutso can be pretty easily handled, unless ALL school personnel are total sheep, including the security guys. At my previous school, there were at least six prior military and two of those were retired SpecOps with combat experience. We all agreed that there would be no students pulling a Colombine at that school. If such began to occur, we agreed to meet at a predesignated rally point, form up, assess the situation and neutralize the problem. Unarmed, until we could get one of the BG weapons. That's not as INSANE as it sounds. Old school design. Lots of tight, blind corners. Untrained opponents. Some teachers kept firearms in their cars on school property, but that was mostly for security on the trip to and from work. I never felt the need and still don't. Even though MS13 is a factor within blocks of my school. Not opening THAT can O'worms again!
Given the sheep nature of most teachers and administrators, I'd have to say that folks in my area would be better off without access to a firearm, unless they would allow ME to keep one of my weapons under lock and key in the principal's office. With me having the only key.
Other places, other areas other states...I'd say it's fine. Maybe even should be a requirement. Certainly even one trained individual with a decent gun at Colombine could have averted the tragedy. IIRC, the school security guard claimed to have engaged in a "running gun battle" with the punks, but he failed to close with and destroy the enemy.