I wonder if you can set up a reward system for her? My dd6 reads at 4th grade level, but over the summer, would rather do other things than read. So I basically bribed her. I told her that as soon as she finished a certain chapter book, she could go to her favorite store for a small prize. Where my dd had claimed she was too tired to read more than a page or two before, after the reward, she breezed through the book (which I knew she could do) in just 2 days. I kept that up for 4 books and now she has to read several books before she gets a reward, but now we both know she can do it. I think the reward gave her a goal to work toward, and the reading gave her confidence and practice, so she could do more.
Maybe you could do something similar with difficult goals for your dd. Do you think the teacher would work with you to come up with something? For instance, after she writes 3 stories she gets a reward. Or for every time she surprises you with information or a skill you didn't know she knew, she gets a token toward a reward.
I do think you should get her tested. My son loved the challenge of testing.
