In any one individual school that might be so, but the comments by the OP, in the end, are general and global. Clearly she would say the same about any kid no matter where they went to school if they had high achieving parents. But certainly you are right that if we were to truly find out that in this one school, these two were the only ones to get C grades, then the discussion, at least some of it, would change some because A) There probably wouldn’t be a normal curve, and B) To the extent there was some semblance of one it would be highly skewed. But we don’t know that about this school (I find it unlikely) and certainly it is not true nationally.
The response of the OP to my point shows that she doesn’t understand probability distributions at all. True, this isn’t a strict probability problem, but the principles apply. And as many have pointed out, in the end she might have zero clue as to factors that might be contributing to the grade results.