Answer:
there are many things that cause school violence.
One of the most common elements is no one listens to these kids who either shoot up a classroom or shoot themselves.
Another one is other students being unneccesarily needlessly and unreasonably hurting others (the victims get it in their head that insults hurt but actions speak louder than words.)
One thing you cannot blame is music or television or media or video games or anything else like that. Believe me those things have very little to do with it. Just because some of the games, and bands, and tv shows express an interest for violence does not mean you can directly blame them when something terrible goes wrong. (oh no your kid and his friend shot themselves and they're fans of Judas priest get them.) sometimes the musician doesn't even have any association with the shooters (during the whole columbine thing they tried to blame Marilyn Manson because they were allegedly fans of his but the sad thing was that during the whole campaign of them trying to promote Manson as some violent heartless satanist the most violent heartless thing was their (media) coverage of the events and when asked what he (Manson) would have said to the two shooters Manson replied "nothing i would have listened because no one else did." so don't blame the media for school violence but rather the schools and the people in it. (i know it sounds crazy and totally out there but think about every instance of school violence what elements are always absolutely there without any uncertainty.