I'm one of those people who is full of brilliant comebacks ten minutes after the situation where I needed them. I don't think on my feet very well. I'll blame it on my poor social skills, courtesy of my public schooling.
It's been quite a while since I've needed to cope with a rude almost-stranger criticizing our homeschooling, but I've got one more "one size fits all" response in my armory, besides the classic Bean Dip one. It goes like this:
"What exactly do you mean by ______?"
In the blank goes whatever the obnoxious person has just raised as an objection. "Social misfits," "qualified experts," "socialization," "overprotective," "independence." It doesn't matter what the noun is -- throw it in there! The beauty of this one, just like the Bean Dip response, is that you don't really have to think on your feet to pull it off.
The typical reaction is a sort of double-take, followed by some backpedaling as the person realizes that they haven't actually thought about it, and probably you have. Suddenly they're on the defensive.
"Well, you know, getting along with different people."
"What do you mean by different people?" [see ... for all you technology geeks, this is a recursive function in our algorithm!]
"Umm, you know, kids who come from different backgrounds, or, well. .... "
"Different backgrounds? What exactly do you mean by that?" [another call to the recursive function!]
"Well, aboriginal kids, or poor kids, or kids who..."
(By now you're thinking, and enjoying as the obnoxious person is madly digging holes and stumbling into them.)
"You don't find any of those out in the community?"