Aimee, Greenville Classical may be an option. I know it's out there, but they enroll 7th grade up in the university model which is 3 days a week. I have friends going there who are very happy with it. St. Mary's goes through 8th as well, but you all are at POP right? My older 2 may take a couple of classes at Blue Ridge CA next fall.
Olivia is a late Sept birthday and has always been the youngest as well. She is always with kids a full year ahead, sometimes more. I see some issues socially at times, but I think it could be personality, aka she's reserved. Academically she's fine, but my dd2 is almost 4.5 and operating at the same level as dd1 was a 2.75. She's a late July and was 2 weeks early and very tiny at 5lbs. I think it's the kid sometimes and Autumn may be just be math/science kid. As far as social/maturity, I think homeschooled kids are just different than ps because they aren't as hardened by all the social junk. They come across as younger, sweeter, and more innocent, at least I see this in the girls.
We should get coffee at the B & N one Tues!