I had the same problem when my stepson was around that age - and too be honest its just started happening again at secondary.
We told him to walk away and not to antagonise the trouble maker - but after your child comes home day after day sobbing, it gets a bit too much to bear and we told him to hit back. He didnt, but like Wee-Janine says, just that thought was enough.
We spoke to the school, i told them how angry i was and what I wanted done, then i stood back and let them follow their procedures etc - when i realised that it wasnt getting any better, i went back in and wrote many letters (Which they HAVE to keep for OfSTED).
You can of course report they failure to keep your daughter safe to OfSTED - or at least threaten them with reporting them...........they soon begin to skip to the beat!
With secondary school, its very different, and at the age of 11- bearly a year from primary school, my stepson was expected to "grow up" soo damn quickly. His head of Year has soon found out that I'm not a parent he can fob off with long and supposedly 'educational' words!
In short, my advice - COMPLAIN, COMPLAIN & COMPLAIN. Nobody has a right to be physically abusive and if your child is in ANY form of danger or intimidation, then you have right to make your opinions and concerns known.
Good luck and let us know how you all get on.
Let your little girl know shes in no way responsible for the situation and that she is loved very much.
Flower xx