A 504 plan may help her very much. It sounds like if she is given extra time, she is able to complete the work in a quality manner and is able to demonstrate her understanding of the concepts and skills. In addition to extra time, it seems she might benefit from technology to help her with her fine motor skills (key boarding). Even private schools must honor a 504 if they take any Federal money.

Learn more about 504's here:
http://www.ldonline.org/indepth/accommodations
http://www.wrightslaw.com/info/sec504.index.htm
http://www.greatschools.org/special-education/legal-rights/section-504.gs?content=868