It's tough. My best response is to lower my voice so as not to sound braggy, to say that DS8 is "pretty bright," and to conclude that our school system is a good one, but that it just wasn't working for our specific kid. Less is more, so I usually change the subject unless the other person actively pursues the topic further.
I do not claim that this is the perfect response. But it seems to meet my requirements of 1) not making me sound like a bragging jerk, 2) being accurate, in the sense of not making my son sound like a juvenile deliquent or as if he's at the complete opposite end of the Bell Curve from where he really is, 3) not sounding like I'm putting down the other parent's school choice, and 4) being acceptable for me to say in front of my child without making me cringe. This is not an easy list of criteria to meet!
It has gotten good response from people I've used it on, so it seems to work. Granted, I don't know what people say when they walk away from me, but I have made new friends with people upon whom I've used this technique, and the neighbors I've told still like us and have my kids over. So it seems to be okay.
<shrug>
