My DD is strong also, and she doesn't cry or get upset anymore, she sort of shrugs it off. I never approach the other parent, or in fact the school. That gives the whole thing too much importance and if they do do anything it will possibly make things worse for DD anyway.
For kids that age it's not really the child's fault it's what's going on with the dynamic in the class, but it would be more kind of the parent guides them to not shut out other kids. I just tell DD that we wouldn't do something like that to someone else but you can't really control what other people do. But you don't have to let them make you feel so bad that it drags down your life.
She had a bad time of it last year when she continued to be friendly to a special needs boy who is our neighbor and was friendly when she saw him at lunch. Some of the kids in her class were nasty about him and tried to get her to stop being friendly to him. The whole thing really upset her and it went on for months before the kids moved on to something else...but in the end, she is still friendly to this boy and is not in that class anymore. She moved on, and learned alot.
