The analogy apparently switched from "A" and "B" the grades/marks in school (hence "A for effort", where A is better than B), to something like the "A team" vs "B team", where the A-team is the elite performers and the B is not as good. Also "C-level" was not brought up until you, but it usually means executives. :)

Although, I see there is at least one person out there who has a concept of a "B-level manager" http://smallbusiness.chron.com/blevel-manager-35848.html so I am less sure about my reading of my grandparent comment.