The Lexile scores are harder to get exact percentiles on, but her range puts her smack dab on the proficient range for a 6th grader.
The best percentile/norms chart for lexiles that I've been able to find is way down on the bottom of this link from Scholastic:
http://teacher.scholastic.com/products/sri_reading_assessment/pdfs/SRI_TechGuide.pdfI believe that it is on page 81. It is reasonable to take her lexile range and look right at the middle of it and find the percentile off of that. Scholastic doesn't break it down beyond clusters of 5 percentiles (i.e. - 90th percentile, 95th, etc. but not 90th, 91st, 92nd, etc.) and they don't list scores above the 95th.
The middle of her lexile range is around 955-960 which would be around the 90th percentile for a fall 4th grader and around the 70th percentile for a fall 6th grader. I wouldn't argue that the teacher should consider her reading at a 9th or 10th grade level based solely on the lexile even though the upper end of it is around the 50th percentile for a 9th grader. The teacher might think that you are stretching it too much.
Honestly, my one kid whose scores looked like that in 1st got to the point where it would be easy to argue that was where her reading was by the end of 1st, though, and I'd suspect that your dd's scores will be harder to argue as nothing way far out of the norm in short order as well.
