IMO, despite this being a terrible idea, there's NO way you can "prepare" yourself for an armed shooter on a school campus. When we had fire drills in middle/high school, we bullshitted. We walked the "safety" route to the athletic field, the teachers took role to make sure every student was accounted for, and we screwed around and class was cut short and we made nothing of it. Then in my 11th grade year, there was an actual incident. A kid decided he was going to make explosives with dry ice in Sprite bottles, which were as common on the grass as students themselves. Had the dumbfuck not detonated one accidentally and mangled his own hands, there would have been nothing done in preparation for a student or staff member picking up one of these Sprite bottles and being seriously injured. When that bell sounded, it was utter chaos. People were running in every direction but the "assigned" route to safety. Students had fled campus in fear and couldn't be accounted for, the staff had a plan according to their drill, but they had almost no control over the situation and it was an actual emergency. You know how it works, someone says "Dry ice bomb" and the next person says "firecracker bomb," then the next kid says "armed gunman with hand bombs and grenades," and by the time it trickles down, you've got a damn nuclear terrorist on campus according to the students. What drill prepares students for that?
I'm not saying there shouldn't be fire/earthquake and lockdown drills, but something like this? Not only is it traumatic for children and stupid of these adults to do, but it's IMO ineffective. We had a debate in class about emergency procedures and why they won't work in a real emergency. Duck and cover under a desk will protect you from falling debris should an earthquake occur. Duck and cover under a desk isn't going to stop you from some unstable creature with a gun; it's not kryptonite. I've seen the footage from Columbine, those poor children were huddled under that desk, which they were taught through drills, and that didn't stop Eric and Dylan from standing on top of those desks and shooting right through them.
I dunno what was on the minds of the faculty members responsible for this "exercise" but I can say that it definitely wasn't the best interest of the students and staff at Scales Elementary.