Violence, even in school, is not necessarily wrong.

People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf. -- George Orwell

There is no firmer evidence that people are morally befuddled than when you have to argue for what should be painfully obvious. To condemn violence as immoral, is like condemning spicy food as inedible, or bitter-tasting medicine as dispensible. (For what Gandhi has to say on this see Was Non-Violence Gandhi’s Ultimate Goal?)

Most educators will readily and strongly assent to the follow statements:

a. Schools are no place for violence.
b. Violence causes violence
c. Violent acts are morally wrong

.
They react generally as strongly, but negatively to the following:

d. Some violence is acceptable.
e. Schools may permit a certain level of violence.

Despite these initial reactions, it takes not very much discussion to bring them around to understanding that statements a, b and c are generally false; and, that d and e are true.
To get down to the reasons, see Permissible School Violence

Cordially,
-- EGR