The thing is you really never know that someone is going to do this. They will usually not tell a soul unless they have a partner or something like Columbine. These things usually happen spontaneously when someone just finally gets to the point or something really bad happens to them.
If you put me back in high school, I could easily point out 20 or 30 kids that were "mentally anguished" enough to do something like that. The problem is nobody wants to deal with the problems of people in problems like he was going through. Everyone will tell you to grow up, stop worrying so much, and just brush it off like it's nothing. What doesn't seem like a bi problem to you may in fact be the matter that puts a kid over the edge to bring a gun to school and kill everyone.
The way I see it is it's something that you really can't prevent unless you change the entire way school is run. You have a lot of freedom in high school compared to former schools. The teachers don't walk you down to lunch or patrol the halls. You have a lot of unrestricted social interaction, and that is what causes kids to break down. There is nobody monitoring the every day struggles that kids go through.
I can see how the transition from some of these schools may just be too much for some kids. It's a pretty big leap from middle school to high school and then from there to college is an even stronger leap. The problem really doesn't lie in bullying, music choice, video games, internet usage, etc. The problem lies with the individual and their own struggle to adapt with the social level that they are surrounded by.