Op am about t do bedtime, so I will come back later if I need to go.
Just to say, I was bullied like this throughout primary school. Similar dynamic (though the perpetrator was the later joiner) and similar behaviour from her. I remember once getting the upper hand with her and her running to the teacher in tears and my burning with the injustice of the years and years of suffering st her hands.
I thunk you are right to pursue with the school on your DDs behalf. Record every incident, including the 'turning tables' event. Other will be able to advise better in the formal process which didn't exist in the 1980s.
Meantime, or as well, I would encourage your Dd to think about / read the dynamics. What is the story of the girl who is driving it? What's her home life like? How does she do at school? Do the other girls that hang off her really look like they are having fun, or are they afraid to, and going along lest her fire turns on them?
Looking back, what drove it for me was jealousy, which worked at a number of levels: I had a happy home life with a stable family, her family was a complicated step family and she got passed around quite a bit. Everything we did at school, work wise, she was good, and I was better. At the end of our second last year I won an award. And I can remember her coming up to me and saying 'my mum will complain to the school of you get made a house captain after this'
In short, what was driving her behaviour was unhappiness at home, driving insecurity, support by a her mum who resented my success. The turning tables tactic is quite clever, and suggests to me that maybe at home her behaviour is being supported and encouraged. Bullies learn their tactics somewhere and it is generally at home.
It didn't get better in my case until we all changed schools. But understanding what was driving it helped me to cope, to care less, to become strong.
Ironically we went on to the same secondary school, and were in The same class for another four years. But the dynamic was different, the spell broken. I flourished, she didn't. I've not seen her in years but from what I hear, I would no way swap my life for hers now, though I wishes that every day of school from age 6 to 12.
Rather, looking back as an adult I feel sorry for her that she felt so bad about herself, that the only way she could make herself feel better was by being so horrible to me.
Back to your DD, help her to use her wind down talks with you to analyse the dynamics of what is going on. It's a useful skill and it might help her to change things herself. My guess is the perpetrator is jealous of your DD in at least one way.
There is a great quote about Prince around at the moment, commenting that he obviously worked out early on what value to assign to other people's opinions, (the right value being : very little)