I taught 8th grade for 25 years, and the kids didn't get rattled by the safety drills, until the one time we did have a kid with a gun. We stayed for an hour plus in that spot, in the back of the room, away from the door with the door locked. Eventually he gave up without harming anyone, at least physically. The kids were hysterical, though.
Until that day they took it as seriously as they do a fire drill, which is not at all. However, my kids were 13 and 14, not the same as little kids.
When I was a kid we had shelter drills, something to do with potential bombs, if I recall from the Soviet Union. I remember hiding under the desk, and there were designated shelter places all around. This was NYC in the 1960s. We weren't scared at all because even then the premise seemed far-fetched.
What these kids deal with today is all too real.
I'm watching CNN and they state he shot his grandmother as well, but her condition is unknown.
God help these families.