Honestly?
A chilly silence and a question:
"It's interesting that you would choose to phrase it in that manner. What led you to think that I want my child to 'struggle to earn a C'?"Then another VERY chilly silence-- as long as it needs to be.
(More than a few seconds and most people are tempted to fill in that silence in a conversation. It takes practice, but
don't.)
Of course, if I were feeling especially saucy, I might just jump right to calling it what it is.
"Interesting use of hyperbole. Did you intend to imply that there are no students in your school who are working to earn A grades, then?"The latter is probably not recommended as a successful means of maintaining a nice working relationship, for whatever that's worth. But it can sure FEEL good to light that bridge on fire and watch it burn if that's the point you're at.

Another of my favorite lines (and one that I
have used in a meeting);
"You say that you "can't" do this. Is it truly "can't?" Or is it really that you don't want to?"If you keep your tone of voice level and calm, that one CAN actually get pretty good results. As long as you make it clear that it
is a real question, which means waiting for an answer.