If someone pulled my kid's hair, I'd instruct them to strike back. That is all you can do if the teacher is no help. She needs to learn to defend herself.
I'd go to the principal, too.
Absolutely not! Most schools have a policy that states that if two kids get in a fight, both are equally guilty. This will only make things worse, not better.
How is the mom sending the notes? I ask because I am teacher. All of our students have take home folders with a daily report. I have had folders come back with the report ripped out. I have had things I sent home not come back, and when I call, the parents never see it. It also works in reverse. There are some students that I do not trust to bring me important communications or take them home. So it is a possibility (albeit a slim one) that if the student is taking them the teacher is not receiving them.
If I were you, I would call the school. Ask to speak to the principal. Have the mother explain her concerns and the lack of response from the teacher. Ask to set up a face to face meeting with both the principal and the teacher.
To let you know, they will be unable to disclose any information about other discipline problems or how they have handled the situation. It may be that the child is indeed being punished, but the school is legally not allowed to disclose that.
I hate to say it, but the chances are slim that they will move that student to another class. At least where I am, we are unable to move students because another student has a problem with them. In that situation, the student that has the problem has to be moved. Ask them about moving her desk so that she is nowhere near the student. Do they have assigned line spots and carpet spots? Ask that she be as far as possible from him in those as well.
Give the school a chance to respond. It may take a few days for action to come out of a meeting like that. Chances are they may already be looking into some kind of "success plan" for that student that you are unaware of and the behavior may already be taken into account. If after about a week's time nothing is happening, then I would consider moving up the latter to the superintendent.
The key is to go in as factual as possible. Try not to get emotional and try not to start accusing as that will put the teacher and principal on the defensive. As much as possible you want them on your side. It will help if you can document specific incidents and when they occur. Finally, are you sure that the girl is telling the teacher? I am thinking of one instance in particular where a student kept telling the parent they were being harassed at school but never once came to the teacher. So when the upset parent called, it caught everybody by surprise. Make sure she is telling the teacher what is happening.
Sorry for the long winded response. As a teacher, I have been on the other side of these types of things. Often there is a lot going on behind the scenes the parents are unaware of. If you have any questions, I'd be happy to try to answer them.