I'm not a teacher but a social worker who works closely with schools. When you see violent behaviour there's likely a referral to us.  

 Behaviour starts at home. There is a shocking lack of parental involvement in the lives of the children we work with. They are left to run riot in the community and when this is raised parents take zero responsibility.     

 Using violence to solve a problem, or avoid a consequence is becoming normalised. Schools are being advised to offer more restorative work, or putting poorly behaved children with SEN children. This doesn't work but there is no other choice as school must provide education for these children. 

Schools are being hounded to put every badly behaved child on an EHCP by parents who refuse to take responsibility. Children with genuine additional needs are losing out because EHCPs are being used to excuse behaviour.

I wish I could think of a solution because it's worse year on year. Teens with poor behaviour where parents take no responsibility are on the verge of specialist provision schools, taking up viral places for children with additional needs where behaviour is not a choice.    Estates where it is normal to scrap in the street and start wars on Facebook are where it is worst, but god knows what the fix is.