If the bullying is taking place at school or is school-related … report it to the school. If it continues, report it again to the school. Encourage your child to keep a record of what happened/was said, when, where and who witnessed it. Schools can deal with bullying if they have the information. Pupils who continue to bully can be permanently excluded eventually. In my experience they usually stop straight away or move on to another victim (who should also report the bullying!).
It is very frustrating when parents contact school to say their child has been bullied for months, but cannot give any names/places or details. I have known parents phone to say there is a bullying issue, but won’t even give their name!!!
As regards hitting back – most schools will punish a pupil who uses physical violence, even in self defence. Far better to walk away and tell an adult, who can deal with it without resorting to violence.
Outside of school – contact the police if it is physical violence or cyber bullying. Also, if it is cyber bullying, print off evidence and then block the bully.