Escape if possible and LEO hasnt gotten there yet. Running from a building where a shooter is into a bunch of nervous hyped up LEO isnt something Id relish doing. Otherwise know where the most hidden unaccesable place in the school is get to it and stay there, be it inside a locker, air vent, any nook or cranny not likely to be noticed. A mass shooter isnt going hunging thru every little corner for individuals hes there because its a target rich environment where he can kill as many as quickly as possible. If all else ffailed keep moving, fast using cover as much as possible. Hitting a running rabbit in a rabbit hutch of rabbits is harder than shooting a sitting one.
Telling teenagers to try to swarm someone armed with a AR unarmed ehhhhh I dont know about that one