Whose job was it to lock the exterior doors?
Upon notification by the P.A., teachers immediately lock classroom doors. I would have to assume that since there was a gun battle at the front entrance to the school lasting several minutes, someone at the front office should have passed the word. We have heard nothing about what happened inside the school other than the fact that the murderer got into a pair of classrooms and started killing.
At some point we’ll hear from others who were there, if they are allowed to speak. I have no doubt the school has told the other teachers to shut up or risk being fired, but sooner or later someone will talk.
I agree, that a couple of locked doors would have made this much less deadly. I have speculated, and someone claimed it confirmed, that the two teachers who died were showing their class a movie, which might have meant they didn’t hear the gunfire, but the question has to be, was the school warned by the office over the school’s P.A. about the danger?
What you describe seems normal, and not very hard to accomplish, for a competently administrated school. Uvalde’s situation probably involves a mix of lapses in planning and foresight, administration, duty, resource allocation, etc., but we can be sure that the whole truth will be elusive.