Well, it's not specific to homeschool, but when someone asks why DS6 goes to private school rather than public, I just say that he has "special educational needs." I think some people have thought that he is at the opposite end of the spectrum, but they're far less likely to delve into the reason!
Just be careful with this. My mom was VERY vague about why DS7 was homeschooled in her holiday newsletter last year (despite my warnings), and people thought DS7 was a troublemaking behavior problem on the road to early prison! Not accurate or fair to the poor kid! And it could have put a serious damper on playdates for us if I had made that mistake...
FWIW, I have better luck with the (brief) truth: DS7 was a good bit ahead of the school's curriculum, and we didn't want to grade-skip him at the time. Some people pursue the issue, some don't, but we've had no bad reactions...and a surprising number of extremely positive and supportive ones!
