It is true that Columbine did have LEOs present, however the argument that an armed teacher or principle could thwart a gunman would be completely refuted by the Columbine incident.
This is the fallacy of gun ownership - it does NOT prevent murder or gun violence.
But the armed LEO at a school adds another dimension. In some states killing an LEO results in the death penalty being invoked. This argument is also weak in that most mass murders end up killing themselves or dying during response to the killing spree. They have total disregard for life.
But if it boils down to teachers and principles being armed, or having an LEO at each school I'd choose the LEO in a heartbeat. During breaks and summer we'd have extra patrolling officers which is also a good thing.
As for assault weapon bans? I agree with it. It's not just the 30 round clip that's bothersome. The AR15 has a muzzle velocity of 3000 ft/sec (compared to a 9mm at 850) - and those bullets can penetrate walls and windows from a good distance. A 9mm is only effective at under 100 yards.
This is why the "training" that a principle or teacher would need would be ineffective. They would be carrying hand guns against an assault rifle - even for an experienced shooter that's a losing proposition.
Lets do several things simultaneously-
Propose automatic death penalty for mass shooting of children (at any location, not just schools).
Put LEOs and walk through metal detectors at all schools.
Encourage (or require) that principles(only) be allowed to conceal carry on school grounds(this helps prevent student access to guns in the classroom).
Ban the sale of assault rifles and 30 round clips
This will not solve the problem (nor will arming all teachers) - but it's a palatable start
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