[Aut Pax Aut Bellum]

Secure the damn school. The kid got in because a teacher had propped open a door.
I am not a big conspiracy guy, but I also don't believe in coincidences. One single door propped open when all are locked minutes before the shooter arrives?

[Responsibility2nd]

Why was that door propped open?

Based on what I’ve read, I believe a teacher left the building after the lockdown order was issued and went to her car to retrieve her cell phone. She propped open the door on her way out and this was noticed by the killer who was outside at the time shooting at passersby in the street. The teacher then re-entered the school leaving the propped door open. Open for the killer to walk in and begin his deadly massacre.

Guesswork on my part. But it seems plausible.