Problems can’t be fixed if people try to cover for a teacher.
Before I retired I worked on a Naval base and was in active shooter drills at least once a year.
There was a drill procedure for each building, exterior door guards were posted, and each interior door with occupants was closed and locked, then whoever was appointed drill leader for each space checked with the building coordinator who was in contact with the base coordinator. This limits movement and access to the shooter(s).
It sounds like this school had no planning at all.
They had plans, they just weren’t followed.
Human error causes a lot of tragedy, probably only second to pride.