I heard about a program in a school somewhere that is DESPERATELY needed everywhere where kids are proactively taught how to vocally side with a child who is being taunted. I am teaching it to mine. They are told to go to the child who has been victimized, say, "Come on, WE don't need to listen to this." and walk away. They don't know what to do on their own and will automatically side with the strongest child, which they view as the bully. This is another reason we should all teach our kids to IMMEDIATELY lash out verbally to accuse the bully of wrongdoing when attacked, because then the kids will see the victim as strong too, and often side with the victim.
I am all for letting your son be strong and fight his own battles, but at this age in this situation that has already progressed, this is what I would do in this situation. I would write letters reporting the child and incidents to the teacher and principal and leave them with them when telling them in person you expect them to enforce discipline with this child. Let them know you would like verification that it is formally reported. I would recommend to them that they meet with the other children in the class to explain the proper behavior during bully situations and that if anyone is caught aligning with a bully, they will be disciplined (if that's even allowed anymore-it's been a lot of years since I was in school and the teachers actually had some power) and their parents notified.
I would also contact the parents of the little girl, and even though I would like to sock them in the face for being horrid parents, I would keep my tone very kind and leave no room for argument or defense. I would state that you know their daughter is very nice, and this is all a misunderstanding, but she has done A, B, C and you have reported it to the school as well. Let them know you expect them to teach her the right way to act toward your son-even if it means staying away from him and that you will continue to make sure the school enforces it. Do it in a letter if you think you can't keep your cool.
I might even see if I could force a mediation with myself, the bully's parents and our kids to force an apology through a friendly meeting and then have the teacher announce to the class they are friends or something..this would take some thought...not sure....something to keep the girl from gathering her minions and continuing the behavior.
As for your son, definitely let him know he has done nothing wrong and that in life there are mean people. He's getting a dose now that will make him stronger. Teach him how to stick up for himself. But don't leave this all on him at this age. These are rules that the school should be enforcing. Good luck!