I would definitely ask the school to intervene. When DD was 8 she saw the beginnings of mean girl behavior on the playground. At first she was only an observer as certain friends targeted friends from other groups but it eventually turned on her. One particularly manipulative mean girl, who DD considered a friend so felt safe from, took control and started dictating who was allowed to play with who and at what times. One weekend we arrived at a local playground and DD was thrilled to find several girl friends from school playing together. She ran up expecting a big hello, hugs, etc. Instead this mean girl stood up, said DD was not allowed to play with them and the other girls were not allowed to play with her. Period. All were stunned but the other girls clearly feared this one girl and didn't know what to do. A couple of them shrugged apologetically and did what the mean girl ordered. A third just stood there paralyzed and began to cry.
Because this was at the playground everyone's parents were there to witness it. The parents of the crying girl were horrified and took the opportunity to discuss that she needed to make her own decisions, not allow someone to dictate being mean to a friend, etc. The girl explained that it had been going on all week at school and mean girl had threatened to cancel the sleepover planned for that weekend if she didn't follow orders about playing with others. The parents obviously would have intervened if they had known. The mother of the mean girl? "I know 'mean girl' is tough..." She was aware but unwilling or unable to address the behavior.
After this incident mean girl started targeting DD during recess and I advised the school asking that she be kept separated from her as much as possible. School SW tried to address the issue with the whole class but conceded it was just the beginning of developing mean girl behavior on the playground. By the end of the year DD was separating herself from the other kids in order to avoid the nastiness.
She also had one particular friend outside of school who literally had drop-to-the-ground-hissy-fit-tantrums if DD ran into another friend at the playground who wanted any of DD's attention. DD and I discussed and agreed that we did not like and would not reward that behavior. I overheard DD explaining to this friend that she had to understand that DD had other friends and she would NOT ignore them if they ran into each other. If this friend reacted this way DD gave her time to stomp off and have her fit and then after a while would go over and ask if she wanted to play. Inevitably the 3 would play together just fine. Now that they are 11 DD has had a few of these types of conversations with this friend and her other friends have learned to tolerate her for DD's sake but none really enjoy having her around. She has no clue how her attitude affects others but since her father is a psychiatrist I assume her parents have done what they can to address it...
