I think they are now saying that teacher went outside, saw the shooter, and called 911. When she came back in, she says she removed the stone that was propping the door open and closed the door, but apparently it didn’t latch. So the gunman was able to enter when he reached the door, because it was closed but not locked.
CYA in progress......
“....because it was closed but not locked.”
I seem to recall them spending hours on that when O.J. was on trial - car door.