Awww hun
My DS is the most non-violent kid i have ever seen. Not one bad bone in his body. He is always fair to his playmates and it breaks my heart when other kids just aren't friendly, caring or sharing as he is.
My DD is exactly the same, and she is constantly bullied. I think that the other more dominant children see her as an "easy target". Don't feel bad for losing the plot, I used to aswell, but after my DH and I would go and chat to her together.

We used to role play, what she could have done. We told her that people will treat you like this for the rest of her life if she allows them to. We were quite blunt. I said that no matter how many schools you change to, or when you leave school, in the work places, there are always nasty people. I believe one of the best lesson we can teach our child is not to run away, but stand up and let these people know (adults can be nasty too) that there behaviour is not going to be tolerated.