You're doing the right things. Girls can be SOOOOO mean! I too was bullied, but by my older step sisters. I shared a bed with one, and apparently kicked in my sleep. A number of times i woke up to being beaten with a hairbrush, or a belt. My parents (Dad and Step mom) did nothing, and I just learned to sleep very still. They did teach me how to fight dirty. I'm not proud of it, but I know how. Always look your opponent in the eyes. Not with scared eyes either. It's not the "Oh S.H.I.T." look. It's the "what?!" look. The 'alpha dog' is definately at work here. All your daugher needs to do, is stand her ground. Have an arsenal of definsive sentences, just in case. Don't back off.
Maybe your daughter needs a good dose of sneaky. Put a bag of chiggers, ants, mosquitos, etc. in her locker, or her pack. Leave a note: "Biology 101" or whatever. No note works too. ( these things must be done ASAP after a bully session. That way, she knows you did it, but can't prove it.. or will be too embarrassed to tell). Oh Mrs. Principal, I'm sorry. I had the ants for biology, and I lost them. I'm SO glad you found them for me.
Get a camera, and have a close friend take pictures of the problem. Then, when you try to present it, it will be in your favor. Also, warn the girl. "If you don't stop ----- such and so will be notified and this (insert your favorite and her least favorite punishment) will happen. If the problem persists, follow through on the threat. That's the most important part. If you don't, the bully will get worse. Holiday cookies filled with pepto Bismol is always a good way to get back.
No offense to your daughter, but it looks to me as if she is allowing herself to be bullied. That's why this obnoxious girl is doing this... because she can! At least teach her a few wrestling moves, so she can turn the bullie into a victim, by pinning her up against the locker. All it takes is a quick twist of the arm and an "Indian Rug Burn". 'Wedgies' are hard to prove, so how will the pricipal know if your daughter did it? Believe me, after a few retailations, the bully will decide it's not worth the effort. The list goes on... and it's good to be a "girl scout" I am trying to teach my boys to be "All 'round good guys".
Tell the bully... "We can do this the easy way and become friends, or we can do this the hard way. I personally am up for the easy way"
I remember as a kid in HS, this asian kid (very quiet) was getting picked on. I also remember he was the talk of the school when he had enough and the bully got his butt kicked by a black belt in some martial art. Teach your daughter defense.... not necessarily offense... but defense is an excellent tactic. Things like wrestling, self defense classes (for street prowlers) etc.
Now keep in mind, I grew up in a sheltered neighborhood. I was tall and stood up straight. No one really messed with me. I was a big "Jock". I read the papers though, and see where kids are getting stabbed and shot by classmates out of anger. Now I am rethinking my advice. I have no idea what your life is like, or your school, other than people need to read the rules. Have you tried talking to the other girl's parents? I'm starting to ramble. Please note, I have no reason to do ANY of this stuff anymore. Everyone I am in contact with is rational.... well mostly.
Good luck and keep us posted. I'm interested in how this comes out.