I was giving calculus just as an example, I was referring to a general subject in school where you have to study and do your homework in order to get good grades. I am pretty sure most of those who got low grades in gym classes did not bother to do any "homework" to make themselves prepared to the next class (e.g. practice catching the ball, do pull ups etc). It's the same with kids who had bad grades at subjects that required thinking, they did not bother to do their homework and were not really interested in reading, studying etc...You should look at the issue from both perspectives