She sounds bored. Does she finish her work early as well? What expectations are on her once she has completed her assignments? Is she expected to sit quietly and do NOTHING or is she allowed to pull out a book for enjoyment while she waits on everyone else to finish?
Get her to be as specific as you can about her teacher being "mean". I'd bet she is not mean, but strict, and is not lenient about letting your daughter entertain herself when she is finished with her work.
Added:
After thinking about it some more, the other mom who mentioned the possibility of giftedness and sensitivity really struck a note with me. My daughter is in a gifted program. She is VERY upset when she feels like she was chastised wrongly. And "fairness" and "justice" are really heightened with this type of child. The year my daughter started the challenge program (one day per week pull-out, where she attends a totally different school for one day each week), she was fine with it. Made all the adjustments, had friends, etc... then one day, out of the blue (seemed to be) asks me the night before her "challenge day"... "Do I have to go to challenge tomorrow?" She ended up crying and getting very upset. She went anyway. I spoke with the teacher, etc. Everything seemed fine. The next week, same thing. The night before, right at bedtime, she gets herself all worked up and crying... still asking me in the car the next morning... PLEASE can I not go?? It took some real investigative work and brainstorming to figure it out: One of the ladies in the drop-off line had said something to her about getting out of the wrong door of the car! Told her she should get out of the other side of the car. That was all it took.
When I thought I had it figure out, I told my daughter: Look. You didn't do ANYTHING wrong. She thinks you would be safer getting out one side, but I have told you to use that side. Don't worry about it. IF she should ever say anything about it again - which I doubt she will- but IF she does... tell her that she needs to talk to ME (Mom). That's it. You are not in any trouble, but if she says anything, just tell her she should talk to your mom.
Do you know, after I told my daughter that, she has NEVER had another issue with going to her challenge class. It had nothing to do with going to the class, but EVERYTHING to do with thinking she was breaking some rule that didn't exist when she was getting out of the car, and worried she was going to get "corrected" about it again. Your daughter might very well have something similar going on. Something about the process of her day might be making her feel like she is being bad, and she may not know that she ISN'T being bad! Simple misunderstandings can really affect some kids.
And my all means, ask the teacher (or go directly to the guidance office and ask them, one of my daughter's teachers was very slow to get on board because it was her last year before retirement I think, and just didn't want to deal with it) and ask about getting your daughter evaluated for giftedness.