Right you are, Terry, we’re all averaged into this together. Sort of.
However, the disaggregation rule is a recent change for ACT. Until 2006, while I was on the board of North Carolinians for Home Education, ACT was very willing to give us reports which included specifics on homeschoolers. You can still see the report for the class of 2005, for example, on ACT’s website here.
That year, homeschooled students in North Carolina earned a average composite score of 23.4, 15.8% above the state’s average of 20.2 and 12.0% above the national average of 20.9. (see page 13 of the pdf report). However, only 2% of the students taking the exam in North Carolina were homeschooled, so their impact on the state’s average was minimal.
The homeschooling data was part of a demographics page which was deleted from the ACT’s 2006 report. I don’t think the private school data was ever pulled out separately, but I wasn’t looking for it. However, ACT is not the first close their eyes — or at least their mouths — concerning this part of the data; the SAT has been unavailable for much longer. Their management told me they didn’t think it was reliable when someone claimed (or presumably didn’t claim) to be homeschooling, so they stopped releasing that information though they still collect it. The latest study I’ve seen of homeschoolers and the SAT used data from 2001.