That's no good. How long's she been at school?
Sounds a lot like my DD about a month or two into a gifted pullout when she was 6 - complaining it was too hard and she didn't want to go, too tiring, cut too much into her day etc etc. I had a word to the teacher who assured me it was definitely not too hard for her, and DD got over it after a couple of weeks. Personally I think it was the shock of being challenged for a change