How Common Are Mass Shootings at US Schools?

The Volokh Conspiracy - How Common Are Mass Shootings at US Schools?

Interesting gun control discussion in the comments (see link above). The salient points are that Cho's weapon was legally owned & worked as designed, so no safety features would have helped, and the campus was a gun-free zone, so the law-abiding (including campus police?) had no access to the best tools for self defense.