Without knowing what you stated regarding school violence, I can only assume you mentioned the main reasons violence erupts in schools. Given that I am sure you mentioned a few of the most common things which breed violence in schools (bullying, social clicks, superiority complexes, and racial issues), I can suggest that the counter argument brings up the understandings for why violence takes place.
For example, you could focus upon the continual harassment of a student, which results in violence, as being understandable. A person can really only take so much before they break. On the same hand, racial conflict can also come to a point where it becomes violent and understandable when people act out against prejudice.
In essence, if your essay is focusing upon stopping violence in schools, the counter argument would state why violence is understandable. On the other hand, if the essay's focus is upon why violence is understandable in schools, the counter argument would focus upon why any type of violence is not acceptable.
Thanks for the answer.It helped me a lot.