I’ll let the firearms debate be played out elsewhere. What other factors might matter?
American youth have different attitudes toward life and death than do
youth in other countries. The authors cited a World Health Organization
study, which reported that American youth are more likely to believe
it’s appropriate to kill to protect their property than were youth in
Estonia, Finland, Romania, and Russia. Similarly, the cited study noted
reports that adolescents in the United States are more likely to
approve of war than were youth in any of those countries.
Here is more. Here is the U.S. trend over time, plus a comparison with Europe. I see weaker social and family constraints, whatever their other benefits, as having dangerous effects on the psychotic outliers.
The good news? School-associated homicides are less than one percent of all homicides involving students. And this:
…trends throughout the 1990s show that the number of school homicides
has been declining. Yet within this overall trend, homicides involving
more than one victim appear to have been increasing.
One politically incorrect interpretation is simply to note that American youth are becoming more ambitious and more "productive," not just in hi-tech. Note also:
…the overall risk of violence and injury at school has not changed substantially over the past 20 years…
Here is an article which suggests the U.S. rate of youth violence is not so out of the ordinary, although it does take different and sometimes larger-scale forms. Here is a more pessimistic (but more statistically selective) picture. Here are further international comparisons.