Is there any empirical evidence for this? It flies in the face of my experience, which is that A students were A students from the start; that C students almost never become A students, and that A students sometimes become C students.
There are plenty of people entirely capable of A marks in most educational situations who don't get them because they don't care and don't do the work. Something changes and they get interested. The jump from an indifferent C to a solid A often isn't that far.
Somewhat related: There are more PhD's than you might expect that were also high school drop outs.