I’m uncertain as to why this exterior door locking system became commonplace, but I can speak to my experience.
In NYC, in my school and in all the schools in which my friends taught, middle and high school students are not escorted through the hallways. When it’s between class periods or a student has gone to the bathroom, or is otherwise in the hallways, there is the potential for a child to enter an unlocked classroom. A child can steal something, go through the teacher’s record book, try to find test answers, etc.
We were NOT allowed to lock our classroom doors when we were in the room with students unless there was an emergency lockdown, etc. On the other hand, we MUST have the door locked when the classroom was empty, to prevent unauthorized entry.
When I was a rookie teacher I had collected money from some candy sale thing we had done. I forgot to lock the door, someone came in and stole the whole envelope with $140, and I had to make that up from my pocket.
Several years ago we had cameras installed throughout the building, which put an end to any mischief.
IMO the time is well past for a sea change. I’m no mechanic but there must be 100 other ways to lock a door that will keep children and teachers safe.
I only scanned the Uvalve report because it’s too painful.