Generally, I think there is a lot of co-oping for the upper grades where parents who are strong in particular subjects teach those subjects to several kids, not just their own. I know in my brother's case they have a very strong and pretty large co-op. From what I've seen in both public and private schools, the teachers (there are exceptions) generally do not have a real strong background in the courses they teach, so in most cases I don't think the homeschooled kids who work with co-op arrangements are at any disadvantage in terms of the expertise of the instructor.
