One reason active shooter drills (ACD) are a terrible idea is it traumatizes the children.
Another reason is ACDs teach kids to be in a lockdown traps them in the same building as the shooter. A kids instinct is to run away from the shooter. Run, run, run is the best strategy. Also unlocked windows for escape routes.
ACDs have been going on for 10+ years now and the shootings still persist and even make things worse. (see below)