Well the question is really why is it such a recent phenomena in the US? Its a very difficult question to answer. Bowling for Columbine tried to answer it (or at least pretended to try to answer it), and didn't really have a good answer.
I don't think anyone can make a case that school shootings are a direct consequence of a lack of firearms. That can't possibly be true as evidenced by the fact that Europe doesn't have these things happen daily. It has to be something cultural. We do have a weird fetishization with firearms in this country. I don't know where that came from. Perhaps it was a necessary consequence of decades of moving west across the plains and living off the land.
I don't claim to have all the answers, but it simply cannot be the case that school shootings are directly related to the number of privately-owned guns.