Aw, the poor thing.
I just want to chime it that I would definately notify the school. Not just her teacher, but also the principal. What your describing is usually defined as bullying and schools take a pretty hard line about that these days. I know something like that happened to my friends daughter at the end of last year. They had the principal and school psychologist come into the class and they all did role playing and saw a video and such about bullying. Now, these kids were in kindergarten so what grown-ups say goes alot further, but the kids in class did respond and things got better.
Another thought would be to sit down with your daughter and figure out if it really is ALL the kids or just some. Girls tend to run in packs and I bet she's not the only one being treated like this. There may be some other kids on the 'outside' that she could be friends with.
My son is also delayed socially. He's much better with 3-4 year olds than kids his own age. We try and have him do things with his peers, but then I also have several friends whose kids are more J's speed. I make playdates with them to give J a break from the social pressure.
Blessings,
Jenny