No, you have not followed the case close enough. That particular door could not be locked, it was broken. Now at the school I taught at we had six doors on the first floor, with one or two doors on each side of the building. So how did the shooter know the exact door that would be open? He didn't work at this school and there is no indication that he had been to the school prior to this event.The teacher purposely left the door open to let the shooter in. She did leave the door open, I do not believe she did it to let the shooter in, I think she did it for convenience.
But you can believe what you want, I think you just want to debate.
The simplest explanation is someone broke that door, someone who had access to the school, and then they informed the shooter which door he would be able to open.

