Thanks, all, for giving me some good things to mull over. Lost my first attempt at responding (argh!), but finally now have a few minutes to redo.

@DeHe - Thanks for your thoughts. I started to include the saga of our many meetings/attempts at advocating, but in the interest of time/space, took it out. Alas, a bit more backstory on that would have made it easier to understand how we came to our current position. The abbreviated story: Although we knew our DS was not typical (e.g., he was a self-taught reader at 3; had a phenomenal memory; had great number sense), we did not realize how atypical he was until he was tested at the end of 1st grade. Behavior problems that cropped up that year (the solution to which was the teacher allowing DS to go off and read by himself during class) resulted in the school recommending various testing. After the testing, we became involved in the DYS program and brought to the school recommendations from the psychologist for acceleration. We were placated masterfully by the administrator of the lower school. Vague attempts at differentiation and enrichment were not enough for DS. There were numerous exchanges/meetings, etc., but the bottom line is that our son was idling and losing his love of learning while he might have been soaring. The school is already aware that we are considering homeschooling, and we have a meeting this week to confirm our intentions.

@bzylzy - I wouldn't want to suggest that all Friends schools would approach a HG+ kid's needs in a particular way. We were drawn to the school because the Quaker influence is so meaningful to us. (How could we *not* want to have our child educated in an environment that promotes peace, kindness and tolerance?) Certainly, though, there are issues of resources and personal philosophies of administration and faculty that enter into the equation when you talk about curriculum and scheduling. In the end, I am not angry; I am deeply disappointed that we could not achieve the level of accommodation/acceleration our son needs. Honestly, I am not sure any school - unless it was filled with similar kiddos - would be a great fit DS. We really hoped we could make this one work -as evidenced by the number of years we stayed.
And yes, we are members of a Friends meeting (the meeting that oversees governance of DS's school). That is one way that our son will remain connected to schoolmates, and we are grateful for that.
BTW, I am happy to have just read The Well-Trained Mind too. It will be the framework I will use for homeschooling DS next year.
Ugh. I am out of time just now, though I wish I could address everyone's comments! For now, many thanks to you all again for taking the time to respond.