For me school has always been about discipline and dedication and not about skill. The school adapts itself to a lower difficulty so that more will pass and they compensate by making it more cumbersome so that the skilled, but not so disciplined, will get bored and not perform as well.
This was also my personal experience. I was pretty good at math but I never got any challanges so I just didn't do anything, but luckily I aced the tests and the only homeworks we got where to have complete the chapters. This translated to other courses where I was good, but I got bored and not motivated so I got a lot worse grades than I could've had if I was dedicated.
I managed to motivate myself later on to get good grades, other would call them great, but I know I could've had straight A grades if I just weren't so lazy and skipped a lot of hours.
Furthermore it doesn't help (doesn't show in grades) if you know a subject really well. Often the workaholics, or a bit nicer: those with more discipline than me, would just study like hell before a test and get a great score, where I wouldn't but I knew the subject well enough so I would get a great score too. I'm a bit upset with this because I doubt that they could remember anything from those tests but I felt I could remember them for months.
On one Calculus test I had, I didn't study or do any of the homework and pulled a ~90% score. When talking to someone who got a perfect 100%, he casually informed me that he forgot how to integrate between tests. I was shocked to hear this, but apparently it's the norm.
I couldn't have put it better, that's exactly what I feel too! It would be so cool if the educational system were designed to help under-disciplined people to perform to their best rather than weed out the ones who can't adapt.