If your daughter consistently scores very highly on her non-math classwork, you might consider advocating for a subject pull-out for language arts. Her teacher will hopefully at least let her read in class if she's already done with her work. If the school doesn't offer to make any changes within the confines of your daughter's current schedule, I'd let it end there and "afterschool" your daughter (i.e. provide some stimulating enrichment for her at home).
The good thing in your daughter's case is that writers are in large part self-made, and she can develop her talents in that area outside of school. Most public school teachers aren't going to be the sort of mentor a budding writer needs anyway.
If nothing else, writing double the number of paragraphs sounds like it would be more productive. :|
