Is it mostly children at school that she's interacting with that you're concerned about, or just her peers in general.
Is she being bullied or just ignored/excluded from what you observe? How does she show being hurt (does she say something, cries, gets anxious, withdraws, gets angry, etc)
She may have a higher EQ than her peers. There may be different reasons as to why her peers and friends treat her this way. 1/ jealousy 2/ competitiveness, 3/ there is always going to be ups and downs in human relationships and your daughter is starting to grasp this fact early on...She may be more sensitive than other and the behavior of those around her affect her a great deal.
These are just some speculations and the causes may be multiple. The approach that is usually most helpful is a multidimensional. You first focus on her self talk (what does she think/tell herself when these things occur. Is she down on herself, does she think that it is her fault for the way others treat her, etc.
Then you work with her feelings. Is she becoming anxious/more self conscious/aware and you help her not dwell on what others do or say but what she's in control of. That way, you strengthen her self esteem and let her see that people can treat others differently at all times for no apparent reason or trigger on her part. You may also encourage her to remain friendly and that those who appreciate her as the individual she is, will be naturally drawn to her. You may have to think of some simple examples of your own life even now as an adult (how some people treat you differently and what helps you when some other ones are not as kind) All along you focus on her strengths and what she's good at. She is learning how to survive in a world that is not always friendly and you're teaching her that she is capable of doing so.
Empowering Your Indigo Child: A Handbook for Parents of Children of Spirit by Wayne D Dosick PhD and Ellen Dosick Kaufman (book)
The Highly Intuitive Child: A Guide to Understanding and Parenting Unusually Sensitive and Empathic Children by Catherine Crawford (Paperback )