I've never found a particularly "good" way of dealing with this-- and it never really seems to go away, honestly. The thing that I've learned about this over the years, though, is that Haters Gonna Hate. What I mean is that there IS no convincing some people, and it's a whole lot more about their baggage than about anything regarding you or your child.
It's taken a long time, but DD15 realizes that now. She has come to view this as a rather convenient litmus test for "people that I might want to be friends with" and she runs everyone through it. If they can't handle the fact that, yeah, she really IS "like that" then she and they probably don't have a lot of common ground.
As for fending off questions, I've found that a simple statement of grade (if it's a classmate expectation) or of age (if it's an agemate expectation)-- never BOTH-- is usually sufficient, and for those who are nosier than that, I turn it around;
Why do you ask?
I'm polite enough, of course-- but genuinely puzzled as to why my child's academics (or birth date, for that matter) are so fascinating to someone who barely knows us.
