I guess my answer would be different depending upon who said it.
If it's casual conversation with another parent (who is not the PTA president or something), I guess I'd just sort of go along with an "It could be worse" sort of comment. If it's someone whom I think I could educate, then great. But I think you have to pick your battles, and this one might well be unworthy of fighting.
If it's a teacher or school administrator, however, I would be somewhat less amenable. This response is an attempt to say that it isn't that big a deal. So whatever your words, your message has to be, I think, that it is a real problem, and it's a problem that could become very serious if not taken seriously. I think you have to make it clear that it's not okay to minimize the problem.
I like ebeth's comparison, just to give a hint of what an HG+ child is up against, assuming you can fit it into a polite, bite-sized form. (I'm not very good at that!) Or I might mention the dropout rate for GT kids, or the rate of depression and suicide. But my goal would be to say something that would make clear that this is not something that should be taken lightly, not something that can be pooh-poohed away.
Maybe start with, "Well, I don't know if I'd go that far. Underchallenged HG+ kids are subject to a lot of serious problems..." Something polite and gentle, but clearly disagreeing, too. And offering info to back the claims of harm.
That would probably be my approach...if I had time to think through my response instead of just blurting out an incoherent "Nuh-uh!"
I'm afraid that's more my usual M.O.!
