I think a big part of the problem is that people don´t care for others anymore. They just look away when they think that they are not affected directly. That way some people can be sure that they can get away with whatever they do. Children and adolescents try to push their limits and when no limits are enforced things tend to escalate. When I was at school, some fighting was normal but there were rigid limits. Noone used weapons even though it was permitted to carry knives in school. Even threatening to use one in a fight was considered to be cowardice (being a coward was even worse than loosing a fight). Fights had to be kind of fair. Bystanders cared enough to interfere if generally accepted rules were violated. That seems to have stopped.
Security efforts may keep these problems partially out of the schools but they are not likely to cure the problem. Letting students go through metal detectors and putting up CCTV are tangible efforts that help the people in charge to demonstrate that they do something about the problem. The violence will be done outside the school. That way the school staff no longer is to blame.
The cure would be to reintroduce some care for other people in our culture. Obviously that´s going to be a painfully slow process without immediately tangible results.