It might sound a bit over the top but the best thing you can do is to get a diary and fill in all the incidents of bullying as they occur. When bullying is subtle and covert, it can take a long time to manage to prove it but the good news is that sooner or later bullies usually overstep the mark and are more easily detected when they do. Talk to your daughter and discuss strategy. Ask to try not to react to anything the bully does for a period of time and record, record, record...... It is very hard to justify one's actions if there has been no reaction. I wish you the best of luck, girls are so clever at this type of bullying.