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.
Math, Physics, and Chemistry are easy to objectively grade, so I'd accept that they tend to have higher grade quality (a notional metric of the correlation of grades to ability) than e.g. literature, drama, sociology, pottery, etc, where the grade quality correlates to the ability of the grader.
Physics classes don't prove a blanket statement regarding grade quality over all subjects, any more than Yao Ming proves that Chinese people tend to be seven feet tall, or the King of Sweden proves that Swedes tend to be kings.
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.