That's bullshit. Maybe it happens in some cases, but do you really think secondary schools, colleges, and universities all over the country are conspiring to inflate the grades of athletes?
Whenever one of my students joins the football team, he now has an impetus to do well (to maintain eligibility), coaches breathing down his neck and stopping by the classroom to see how he his doing, plus as a teacher, whenever he is being lazy, I can just say, "What about football?" and he will get right back to work.