it’s all about damage mitigation. you want to be away from the worst overpressure and glass, then, in a part of the structure likely to remain standing. you’ll probably still be thrown around and hurt, but reducing the kids who died from flying glass and covering their head will mitigate a lot of potential trauma.

now, depending on the winds and what kind of weapon was employed, mitigation may be silly, but you always train to do as much as is practical.

Also I think these drills - growing up - were actually smoke breaks for the teachers.