That is a difficult question to answer. Not knowing the family or peer influences makes it difficult. But, I can tell you that the influences include abuse, bullying, teasing, criticism, lack of support from the family and peers who are too afraid to stand up to a bully.
Being new in a school, different culture, speaking with an accent, clothes and shoes not "cool," just being different that the bullies who are already established can cause a child to lash out in anger.
Violence can breed violence. Teens and pre-teens need parenting more than they want and most parents realize. Children do not understand, nor do they have enough information to make critical decisions about their actions. They live on instinct and self-rationing. Without outside influence and more information, structured guidance and good parental role models, bullying is a defense. If I beat you first, then you cannot beat me. Lashing out is an intimidation tactic that works. New tools need to be established. Taking away the one that works without replacing it will not work. It goes underground.
I hope this helps Jangaplanet. Good luck.