Your post brings up so many truths. One is, a person with a semiautomatic in a public school building owns the hallways, owns wherever he/she roams. He continues to shoot and kill until one of three things happens. He stops. He commits suicide. Someone else stops him.
A public school is a soft target, killers know this, that's part of the problem. The old slogan, the only thing that stops a bad guy with a gun, is a good guy with a gun, actually holds much truth in this scenario.
The other slogan that holds much truth is the answer to why a person needs to carry a gun, or in this case, why should there be trained persons in a school with a gun...answer being, when you need a gun, a 911 call and LE are only minutes away. I guarantee in 3, 5, 7, or 10 minute response time, lots of mayhem may occur.
Students and faculty are sitting ducks, helpless, every time. The only choice is to lock your door and hide. Some more recent training goes beyond the lockdown theory, and is beginning to give power back to faculty, power to choose to run, lockdown, and/or fight back.
I made the argument for rubber bullets, non lethal, at least a person would have something to shoot at the murderer. This kid stopped when a man with a gun confronted him. This is an important point. A shooter likely will attempt to take cover even with rubber bullets being shot at him, allowing time for LE to arrive, or even potentially ending the threat.
"A resource officer confronted the shooter, who immediately surrendered to the deputy and was taken into custody,"
https://www.cnn.com/us/live-news/apalachee-school-shooting-georgia-09-05-24/index.html
If the rifle would never have entered the building we'd not be having this conversation. I know, lots of other murdering possibilities exist outside the building. But preventing entry of firearms in to the building, and having a few trained and armed folks inside the building, moves the building out of the 'soft target' category.
Lastly, none of this happened in a vacuum. I always said, and continue to believe, (and much of this is beginning to surface in mainstream media) my opinion, LE, FBI, Counselors, School personnel, Child Services, Probation....one, or several of these entities had involvement with this murderer over the years. He had to enroll in that new school...that means he, or some parent/guardian, had to show up there to fill out enrollment papers. I don't know Georgia law, but in PA, you don't start school until you have proof of immunization records, and a students file is sent to the new school from the old school. I opine this kids file folder is a thick one.
Sorry for the long post