A teacher at the school is disputing the authorities timeline that claimed a door was left open, says door was not left open.
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.