We can avoid that by stationing armed police at the entrances.Not really. Almost all school shooters expect to die and aren't deterred by death. Your solution at best cuts down the casualties while doing nothing for the frequency. At worst people can't tell the difference between the shooter and the students trying to stop them and you get friendly fire. And a smart school shooter will start at the armoury to cut people off from it.
Another major problem is by putting an armoury on the school grounds you allow people to get "crimes of passion" style events where a student gets emotionally compromised and goes to the armour to solve their problem. Or they decide to play a "prank" with a real gun and tragedy ensues. You'd also get a massive upsurges in suicidies since handing moody teenagers guns is going to result in this. Crimes of passion and suicide are major causes of gun related death, and I fear they'd happen on a far large scale than school shooting if you make guns more easily accessible on school grounds.
The 1st amendment doesn't apply to those types of weapons. It states firearms.Automatic grenade launchers should be mandaory for good guys so they can overpower the bad guys. It’s much better than your communist solution. I’m so disgusted that you want to share weapons between everyone like in the Soviet Union.