I left the UK for Canada in the hope of a more gentle way of life. Three days in, and there was a school shooting up the road in Scarborough (near Toronto). Apparently it's not the first this year either. It only goes to show, wherever there is access to a gun/knife culture, you get this sort of tragedy. Since (I would argue) we largely learn by copying others, certainly when we are young, then the idea that problems are solved by shooting must come from experience. And as a kid that experience comes from popular culture. So yes, I do think that TV, movies, computer games, and parental tolerance all feed into it.
Before anyone mentions it, I know that guns have not always been available to kids, but knives have. As a kid myself, I carried a pocket knife, and I had plenty of access to kitchen knives; but no-one in my school was ever stabbed, or as far as I know, threatened with a knife. That was 30 years ago, and clearly something has changed. And it's not weapon availability that has changed, it's the idea of using them to solve trivial and stupid arguments that has. As I said, popular culture.