Is the Head also the teacher of your DD's class? If not I think it might be helpful to ask for a meeting with the Head, the class teacher and the senior mealtime supervisor so all bases are covered and a single approach can be used to manage the behaviour of the girls who are causing so much upset.
I'm trying to think what I would do as a teacher if a parent came and told me that this had been happening (although I'd like to think that I'd notice!) but as I've never taught in a small school I'm not sure....
I think I'd rearrange the class according to children's requests - I actually do this anyway; let each child write down the name of 3 people they work well with and a different 3 they are friends with and I say I'll do my best to put them with one person on their list. I give them an option to come and speak to/e-mail/leave a note for me if there is someone they would feel unhappy about being placed next to. This is quite a telling exercise in itself in terms of class dynamics. I'd then position the children uber-carefully so that the probable ring-leader was in a position that meant she could not make eye-contact with her cronies (at the very front or very back) whilst positioning the 'got at' children (your DD and the two boys) in positions that would give them lots of options for people to talk to, good eye contact positions with their peers but also with me. I'd also devise a lining up order for whenever the children go to assembly/line up in the playground/into the dinner hall etc. so key characters were split up and others supported by friends.
At the same time I'd set up 'worry books' (I've done this and implemented it across the whole school) which are notebooks that every single child writes in every single day after lunch and the teacher reads every single one ASAP (silent reading time for the children). If everyone writes/draws in it daily, even if what they write is "I have no worries" then it is not possible for others to work out if they're being 'told on'. It's an effective way for children to inform teachers of possible bullying behaviour and means it can be dealt with immediately. It also helps that the upset is written down as it forms a record and means any patterns can be identified but also gives an opportunity for the teacher to reply. I have often written in Y4+ worry books, "would you like me to speak to X about this?" or "let me know if you'd like me to help you sort this out". There are potential child protection issues with it so I always make it clear that although a teacher will never show one child's worry book to another child sometimes we may need another adult to see it to help sort out a problem and I always ask a child before I show their book to another member of staff (I usually get them to take it to the other member of staff so they are the one doing the sharing - it can help children feel like they're being taken seriously too which is no bad thing).
Once I'd completely and utterly manipulated their in-class movements I'd work on lunchtimes/playtimes which would probably involve something along the lines of sending 2 children per day to 'help in the office' (I have actually done this - they love shredding paper, delivering registers, sorting school newsletters etc.) and I would be very careful to separate the ringleader and your daughter as often as I could so that the ringleader couldn't see who her other friends were playing with whilst she was occupied.
Meanwhile, I'd be speaking with the mealtime supervisors and asking them how things such as the bars were monitored and perhaps find out if a more concrete system could be put in place to ensure turns were taken fairly.
I would probably also suggest to the parent of the child that they consider moving to a larger school but would warn that girls aged 8+ often have friendship issues and that there's no guarantee that moving school would be the answer. I'm not sure how helpful my reply is but I thought I'd explain what I'd do if your DD were in my class - I detest exclusive behaviour of any sort in school though and would do everything I could to turn it around and would very definitely make sure I didn't let the ringleader choose teams but would come up with a fair and transparent system so she didn't have a chance to leave your DD out in class. I'd probably end up being complained about by the ringleader!