How long is initially? I think there can be a lot of mobility (both ways) between B and D, with maturity / maturing being a major factor.
A true A is likely to stay an A, barring bad luck in the form of immaturity, broken hearts, health issues (esp mental health issues, several of which often surface at this age), etc. That can be true even if they are the very big fish in a too-small pond.
Es might be more variable, depending on why they were otherwise underqualified. There are athletes whose grades wouldn’t otherwise qualify them for admission b/c of their schedule who can move up sharply - and athletes who will drop out b/c they can’t hack it.