3. She was not there in the initial moments of the shooting. She arrived after a perimeter had been set up. No one is allowed to cross the perimeter because it will only add to confusion and likely cause more people to be shot, including by the police who are not going to understand why their perimeter collapsed and now they can't focus on the active shooter, and have to deal with parents running around the school.

You're applying logic to an emotional argument. The point is not that in this specific aspect the police were following procedure (they were) but in driving home the visual that the police will refuse to act while preventing anyone else from acting.

If they had held parents down while officers were moving to take out the shooter, it would have been justifiable and correct, and there would be no emotional resonance with people who respond to that sort of thing... which is almost everyone who doesn't think children should fade away in an ambulance after lying in a pool of their own blood for an hour while the police literally sat on bike racks and car hoods outside.