One of the concerns schools have about part-time homeschooling (and this even applies to private schools) is that they will be held accountable for the results of instruction that is largely out of their control. It is easy to view this as controlling behavior in the sense of infringing on privacy rights, but you also have to think about it from the standpoint of consequences that the school and school personnel take for situations over which they have no influence. Any student who is on the books in the state count as being enrolled at a school affects their attendance, graduation, and state testing figures, which feed into their school and district performance figures, which affect Race to the Top funds, state receivership/supervision status of the district, NCLB accountability numbers, etc. Every public school administrator I know (and a few private school admins, too) has at least one directly-observed horror story about a homeschooled student who was not receiving adequate instruction, or was nominally homeschooled as a cover for some truly neglectful/abusive family situation. Granted, there is a fair amount of selection bias, as the successful homeschooling families are probably less likely to wash back up on the public school shores than the poorly-homeschooling families. But the point is, their anecdotal experience causes them to fear that a student being educated outside of their own (presumably) watchful eyes is not going to learn, and then the school will be punished for it when mandated testing time rolls around.

I have had more than one administrator say to me in surprise, "You homeschool? But you're so normal!" (Little do they know!)

...pronounced like the long vowel and first letter of the alphabet...