Yes, I know of one particular case that I've seen first-hand here in town. The parents are both VERY intelligent (one HS teacher, one college professor) -- and they actually make an effort to teach their child things outside of school. Their son (who was diagnosed with down syndrome) is an intelligent boy who actually isn't that far behind his class, especially compared to the others in the school with down syndrome.
My experience from what I've seen locally is that most parents treat the schools as more of a daycare than a place for their child to learn. The aides that work with the children with down syndrome also express this same attitude. I think the boy I mentioned above benefits greatly from having his parents teach him in tandem with what he is learning in school. It's surprising how normal and capable the child is on his own.