How can a teacher who isn't a gun person be expected to fire at a target that is moving amongst hundreds of kids?, he is responsible for every shot he fires. So what happens if the teacher starts shooting without knowing where those rounds are going to end up. He or she could easily kill more kids than the shooter. If these rounds go through walls and kids are hiding behind them, then what. This is a stupid thing to consider, you need sniper quality people to shoot into crowds and hit their target.
We all know many cops can't shoot for crap, so if you give them the same training the police have, in some places that is little to no training. Plus Reloads and firing on the run, it's going to be a mess. Imagine when a teacher hits another student by mistake, because he didn't know the bullet flight or time it would take to get there. I would not want that job, with too many moving parts. Treating them like what used to be called 600 schools when I was kid, is a better idea. One way in and one way out. Put your metal detector there and 2 guards, these new schools have too many entrances and exits, along with too many buildings, they need to build them higher instead of wider. It would take time but forget the beautiful campuses' and concentrate on safety.
You can also have emergency doors that open if there was an attack, to allow everyone to escape, but are locked during normal hours.