M.D.
I may be in the minority here, but at this age the most important thing should be - is she learning to LOVE learning?! It doesn't sound like it. Whether it's too much pressure, expecting too much, whatever, I think you are running the risk of really turning her against school long-term. If school is a source of frustration and even a source of discipline at home, she is being set up for school to be a negative. She may even have quit because she feels she can't be "good enough" anyway, so why try?
School should be a joy, especially at age 6, full of wonder and excitement. Learning should be fun - yes, it's hard work at times, but usually it should be fun! A D for the day?! Ugh, what a negative impression she must be getting of school and if you are also "working with her at home" she is learning that school means work and homework, not learning and exploring new things and new ideas.
She has 12 more years to sit at a school desk... I'd be very careful about how she is encouraged to view school and learning. I don't mean to suggest that a 6 yr old shouldn't be expected to try her best or that she should be only playing, not learning.
Last year, I sent my daughter to a very "academic" pre-school and could see how it wasn't fun or joyful. I changed to a completely different school with a much more play-based program. I now have a child who at 4 1/2 is reading on a 2nd grade level simply because she LOVES to read. We never "worked" with her at home, we just read a lot and try to make learning fun. Now my son will probably not be an early reader like she is - he's just a different kid, but my main goal is to foster a love for learning.
I try to relax and back off and let them learn at their own pace. It's hard to do sometimes - we want our kids to know facts, know math, etc... I try to remember that kids are learning SO many things at school besides measurable academics. A child who loves to learn will fare much better than a smart child who is pushed or forced to learn. Just my two cents! :)