[Svartalfiar]

At this point it seems that one teacher compromised security by propping open the back door so she could skip out to her car. I speculate that leaving your class for unofficial breaks is not allowed, and she propped the back door so she could get back in without the front office knowing. It only takes one selfish idiot to destroy a security plan. I couldn’t live with having done something like that.

That seems unlikely. Sounds like the guy sat outside for 10-12 minutes popping shots off, so why would anyone go outside at that point? She certainly didn't leave before he got there and sit outside for that long? Did she somehow forget to close the door on her way back in earlier?

[Chewbarkah]

In an official briefing, a colonel in the Texas DPS states that at 11:33 the teacher was standing at the back door when a LEO “sped around to the back door, thinking she was the shooter, only to realize the person he saw was the teacher.” (My paraphrase). If the colonel is to be believed, at a point when the shooter had ripped off lots of rounds and LEO was on scene, rushing around searching for him, this teacher was standing at the OPEN back door. Despite what would seem to be clear and present danger, she apparently walked away without bothering to close and lock the door. Minutes later the shooter entered the open/unlocked back door and started murdering children. I’ve wracked my brain for anything to explain this; “forget to close the door” is not plausible.