How to get defiant 8yr old to behave

My 8 year old daughter goes to a private school and is very defiant and disrespectful. She can go a couple of months and be great. Then all of a sudden her attitude is horrible. We have grounded her and taken all of her electronics away. Nothing. We have tried sitting her down and talking to her about what she is doing wrong and she says she will be better tomorrow. The next day comes around and she is still defiant. We have also tried telling her if she is good we will reward her with something. She just doesn’t care. We are at a loss on what to do. She is at risk of getting kicked out of her school, and we really don’t want that. Its a very good school. I asked her teacher if she is having issues with any kids and she said no. She has her friend she always hangs out with and they get along great. So I dont know what’s going on. Is there any other way to get thru to her? Or what else could be wrong? She goes to sleep at 9pm everynight and her teacher says she dont seem tired at all. She just always back talks and doesn’t listen. She got sent to the principals office and went back to class to go right back to the same attitude. I just don’t know what to do anymore. Any advice or recommendations would be greatly appreciated.