Seriously? Do you really believe that? Given a room of people do you honestly believe that in talking about, let's say Math, or Physics, or Chemistry that you couldn't immediately identify which of them had grades in the top 25th percentile versus the lower 25th percentile? I bet I could _even without having studied those domains_ in 20+ years.
Now, I'll agree that grades don't correlate _perfectly_ with ability for _everyone_ - we all know the bright slacker who doesn't turn in his homework, and maybe skips a test or two. But, in general, a student who score 95% in physics knows their material very well. A student who scores 15% in physics is probably clueless.