Playing a D1 sport does not necessarily imply a hit to grades. I actually had slightly higher grades in-season than off-season because during the season I relentlessly prioritized and eliminated non-productive activities.
Those who don't play a D1 sport or are off-season have required activities that in-season D1 athletes are exempted from and participate in other voluntary activities offered at USMA. The time requirements for Corps Squad athletes are more than for most other cadets, but the difference is not as great as it might first appear - USMA has a way of filling the free time cadets think they have.
Grades are more a function of ability and study habits than whether or not one plays a D1 sport.