The shooter makes it into the school under pursuit and enters a classroom with two adjoining classes.
Officers are reported as shot in the process.
At that point, there is an armed gunmen in a classroom barricaded in.
It's now a hostage situation.
The rest of the school is occupied by police and evacuation/threat plans are being followed.
Storming a classroom filled with hostages is not procedure.
Parents and others running into the school when they already know where the shooter is and what the situation does not help anyone.
Although the timeline is 90 minutes or so from entry until he was neutralized that doesn't mean he didn't just shoot everyone in the first minutes.
The idea that they just gave up or were all cowards is a bit irrational.