I agree with the above by
2ndsojourn in that we have a violence problem in this country. For whatever reason, the youth in this country just seems much more violent in general compared to previous generations. I'm just under 30 years old and don't remember kids being as violent as they are now.
For one example, I would use "The Knockout Game" where groups of (generally teenagers) are hitting folks at random, in the back of the head in order to knock them out. There are videos of this all over youtube.
I must however site a reference to the 'school shootings' in the United States and across the world. It doesn't just happen here. Please bear with me, I know its the wiki, but the basic info is there. It shows that school shootings happen in countries where you can't (reasonably) own a firearm...
http://en.wikipedia.org/wiki/School_shooting