As a teacher I am just
at the parental perspective from almost all posting on this thread!
Please teach your child never to hit anyone and to tell a person in authority as soon as someone threatens them.
Teachers would find it a breeze to deal with bullying if we were actually made aware of it! The minute children try to sort out bullying issues themselves, situations escalate like wildfire until you have dozens involved including parents.
The worst, worst thing about bullying is feeling unprotected, that no-one cares, that you are unsafe, that the world is a scary and unfair place. Tell an adult and the situation will quickly diffuse. Yes adults sometimes deal with bullies ineffectively, but it is always, always better to let a person in authority deal with it.
I'm just so
though that the MN consensus is that it's good to teach your child to hit back and not to walk away and tell a teacher! They will not be seen as victims ime but it will prevent a violent power struggle which will never be resolved.
I have a middle class, lovely student in Y8 who is an occasional victim of groups of dickhead boys. He punches them back occasionally and they find it entertaining that they can get a rise from him. Don't let your child be that boy. He is confused that it doesn't seem to stop them but hadn't told any adult it was happening! I begged him to tell me the names of the boys involved so they could be hauled over the coals but he would rather try to sort it out himself 
As someone upthread said, it's rarely just about the physical side and if a child is walking in fear and reliant on having enough physical strength to defend themselves, it's not right.
I think of schools as mini versions of society itself. If people in society meted out their own justice, where would we be?! Someone breaks a law - tell the police. Physical assault is no more acceptable in the playground than on the street.