another two things i do not understand:

-why cops never enter the buildings, always waiting until nothing is moving/making sound anymore (read: everybody is dead already).
This especially striked me when i watched documentaries about Columbine, 3 and more hours before SWAT moved in.

-the retarded policy of ducking/cowering under a desk (seen in Columbine and i guess same happened in Virginia).
I think is MUCH better run like hell and try to escape, especially against gunmen armed with handguns only (not Columbine case, but still...).