A couple years ago, the school I used to work at started up a new safety program that was pretty extensive. We had 5 different procedures for different situations--shelter-in-place (tornado), evacuation (fire drill), evacuation-relocation (bomb, gas leak, something wrong with the building, etc.), soft-lock down (doors locked, but continue teaching), and hard-lock down (doors locked, lights out, hide, etc.) They also installed a buzzer system for the front door.
We needed specific procedures for these situations because we had several issues with unannounced lock-downs that all turned out to be false alarms. With one, there was a gym class outside and we didn't have a procedure for what to do if a lock down is called and your class is out of the building. Another happened at lunch time. The lunch room is in a multi-purpose commons space that is right near the front door/office. That was absolute chaos, trying to herd 300 kids into nearby offices and the music room to hide. We had several practice lock downs during lunch after that.
ETA: The school was not in a high-crime area. But generally, things get done in schools that have vocal parents and this school (and district) has LOUD parents. If parents don't buy in to what the school is selling, it generally doesn't fly.