My son went to a small private school for 6th and 8th grades (consecutively due to a skip) that was very supportive of acceleration, both whole grade and subject, and independent study. They claimed to be supportive of differentiation as well, but it never really worked out, partially because the teachers were already overworked and partially because my son didn't want to stand out any more than he already did.
My son also has dysgraphia-like symptoms but no ADHD. We found that because there were no advanced options (like honors classes), everything but math, even with 2 whole grade skips, was just more work without being more advanced. Even the math, where my son was accelerated 3 years, was not enough because it was geared toward average learners.
We ended up homeschooling for two years after 8th, and are now doing part time enrollment at the local public high school.
Perhaps you could see if they would let your son do an independent study for English?