High School Shootings
First off, guns are not legal for Highschool kids anyway, so that argument doesn't matter. They would have found a way to get a gun if they really wanted it.
As for the reason as to why this happened, I don't know, but I do have a few thoughts. I think two kids in Columbine went over the edge, further than any other kid had done before, for what reasons only god knows. Before that time, taking a gun to school was generally not a consideration by kids. But since the idea had been put in their heads, it would be used again, and is becoming more common. The more something gets done, the less shock value it has. With less and less shock value, it becomes easier to convince yourself to do something crazy like this, especially in a jacked up teens head. Further, parents are refusing to take their responsibility in this matter by admitting that they didn't see their crazy kid heading towards this path. When I was growing up, my parents knew if I had a bad day at school without me having to tell them, they watched me, were part of my life on a daily, even hourly bases, and thus could see changes in my demeanor and behavior. If more parents did that and were as involved as my parents were, this wouldn't happen. However, there will be some bad seeds, and they intern sowed the idea, which started the wild fire.