Bit of background: DD was best buddies with X for the whole of Reception and the first term of Y1. In the second term of Y1, Y joined the class and the three of them became a tight-knit little gang for the remainder of the school year. But something changed not long after the start of Y2 (i.e. a couple of months ago) - according to DD (and obviously I only have her side, though I haven't heard anything to contradict it yet), X and Y started excluding her, picking on her, with Y in particular seeming to lead things (e.g. Y told Z, who's good friends with DD out of school, that she wasn't allowed to play with DD any more and if she did, Z could 'never come to Y's house again').
Things seemed better for a week or so after half term, but this week DD's teacher said DD had been mucking around with X and Y in class, talking back etc (not like DD, and I think not like X and Y either). Then today her teacher took me aside to tell me DD had scratched Y on the face after Y had said something that upset her. DD told me later that the 'something' was Y taunting her that she always got more sweets when she went to Z's house, which seems (and is, effectively,) very minor, but the subtext was that Z likes Y more than she likes DD, which upset DD since she and Z have been friends since they were in nursery together. She also said that Y often makes a point of telling a teacher when DD has done something slightly naughty - e.g. DD threw a friend's jumper behind a fence and the friend didn't mind, she and DD were laughing about it, but Y went straight off to 'tell on her' even though they'd got the jumper back.
Now, I'm not defending DD's behaviour at all, and told her very firmly that she cannot scratch/hurt people when she's upset, no matter how they make her feel. She does have trouble controlling her feelings and will lash out when angry/upset, so it's an ongoing issue and something we're trying to work on with her. On the other hand it seems to me that Y does try to provoke her, for example I suggested to DD that she simply stay away from Y, but she says that when she does try to avoid her, Y follows her around asking her why she won't talk to/play with her etc.
God, sorry, this wasn't meant to be so long! So - what do I do? To complicate things further I get on really well with Y's mum, who is lovely, but I'm going to have to talk to her I think - not least about the scratch, and also because DD was planning to have a birthday sleepover with X, Y and Z in a month or so, which is looking like an increasingly awful idea. If you have read this far, thank you - and any advice or insight would be much appreciated...