From what you write, I don't see any signs that your daughter is being bossy. Though reading the assumptions that others wrote made me think it's at least worth making sure she is not sounding bossy. Even if you objectively think that she is not being bossy, make sure she is gaining the skills to ask, not tell others what to do, etc.- just in case!
And yes, unfortunately, the snappy tone is quite normal at this age, but that doesn't make it ok, and it doesn't mean every kid talks like that. And you see that it hurts your daughter's feelings, so it's ok to teach her to stick up for herself.
As others have mentioned, work out age-appropriate social skills and practice them with her. There are tons of books with these topics. The best thing you can do is role play with her to make sure she knows how to stick up for herself, but also how to talk to others- take an interest in other people, take turns, etc.
The hardest part of this all is that you had to sit and listen to these girls treat your daughter badly. I volunteered a lot in the classroom when my daughter was that age and I hated hearing how the kids talked to one another. It made me aware enough to do just what I suggest you do- make sure my daughter was not doing anything to provoke others, and making sure she knew how to ask others to not speak a certain way to her. I used to actually envy the moms who weren't there to hear it all, because the fact is this is so common, but mostly because all of the kids usually turn out just fine, it's just excruciating to witness these interactions!
So try to take it in stride, mama. Your girl will be just fine. Make sure she is prepared for different social situations at different ages, that is the best you can do- along with just being there to hug her. This is probably a lot harder on you than it is on her! :-)