I graduated in 1986, and the only time you saw guns in school was on an ABC Afterschool Special, or on a "lesson" sitcom (i.e., "Diff'rent Strokes," "Facts of Life," etc). No one was dumb enough to do it, though.
I wonder if part of that problem stems from neglect at home and the whole, "you can't fight back" thing. Little kids used to duke it out on the playground - and while that sounds bad, at least it didn't happen often. And when someone complained about a bully, there were consequences for it at home. Fights for little kids at school was normal, but then the principal knew parents would correct their children at home, while explaining why that behavior wasn't a good idea.
Nowadays, no one is at home or no one has time to discipline the kids. The school is supposed to do it now. So when you have child with a lot of rage inside of him or her, and they don't know how to deal with it, they can't fight it out, they can't tell anyone about it, they get in trouble for writing it out creatively and the only message they hear is "tell your teacher" (i.e., snitch and get a bigger beating for being a coward) I can see why they explode in violence. Schools should invest in more clubs that help kids work out their anger creatively - like boxing, karate, track & field, etc for kids who *don't* make the teams. I think it could help.
Me, I used to write really violent stories in high school. Today, I would've been locked up and labeled for it. I think the problem comes from several sources, but it starts at home.