I'd still bet money on the C students being mostly just lackluster students.
Yes, I'd agree. But on the other hand, it might also be a reasonable bet that most geniuses receive poor grades. Certainly a surprising number do.
The system hugely values conformity. I remember in high school, I figured out I could get good grades in a given class by playing to the teacher's own idiosyncrasies, especially in humanities subjects. I remember English teachers in particular would always have their own special pet peeves and likes,
and as long as you took note of these, and basically agreed with them, then you would probably do well. Sometimes I thought the teacher's priorities were stupid, but if I didn't pretend like they were important, I would be penalized.
Although it seems like this wouldn't apply to the maths and sciences, it was also true in these classes, to a lesser extent. It just wasn't something you had as much control over. For example, one math teacher might hugely penalize careless mistakes, but not give out very difficult problems. So you had to be really meticulous with trivial arithmetic, etc. Another math teacher might challenge students more , but would not severely penalize trivial mechanical errors. You can get used to the priorities of the teacher, and then adapt.
But I find that often people who are really creative and passionate are less likely to submit to a teacher's wishes, and are more likely to be 'in their own world'. Of course, I'm just talking about probabilities. Creative people could still, on the whole, be willing to please authority.