Yes, mental illness is the major cause of school violence, because mass murders are all perpetrated by people who are mentally ill. The Sandy Hook perpetrator was ill, as was the Columbine shooter. You would have to be mentally ill to some extent in order to justify doing something so terrible to all those innocent people.
Just because someone carries out a mass murder whether at school or elsewhere does not mean they are mentally ill. They may understand the difference between what's real and what is not perfectly. They may have full control over their actions. BUT they may decide to do evil anyways. Those people are not ill and suggesting they must be only makes it easier for them to trot out that excuse in front of a judge and get away with going to an insane asylum instead of prison and then getting released when they have conned doctors into thinking they are better.
Some people really are ill. Those people have my sympathies. Hope you get better. But most people who commit violent crimes are just choosing to be violent and shouldn't be given the mental illness card to get out of jail free.
This question is one that has been around for quite some time and is not easily answered. There are groups on both sides of the issue that will insist that they are right and they will not budge regarding their opinions. Both sides have quality arguments and they both need to be open minded.
The scenes of school violence that make the news are ones of mass shootings like those of Lanza in Conneticut, but most school violence comes in everyday portions of gang violence, teenage boys being violent, and bullying. It is a small minority of cases where mental illness is the cause of school violence.
Mental illness is not usually the cause of school violence, or any violence at all, for that matter. Of those that act out violently in school, only 4 percent have a mental illness. School violence is usually a result of a kid being pushed to the brink after years of torment whether it was at home or school.
Most often there is a mental dysfunction that occurs that leads to a student becoming violent. However, dysfunction is not mental illness. Mental illness is something more permanent. Dysfunction is an irrational reaction to a difficult situation. There are often things that lead up to this reaction. Understanding these things and identifying this before the behavior before a violent reaction happens is where the focus needs to be.