I do disagree; children shouldn't be graded on how well they can perform athletics. If they are graded, it shouldn't impact their GPA, their ability to graduate, or their ability to pursue higher education.
If you made athletics a part of a requirement to have good grades, that is at the very least discriminatory against those that do not have the same kind of physical abilities. Judgements like these are hardly justified in a modern society where mental ability, creativity, entrepreneurship, and concentration are just a few of the key factors in the success of a person.