Mr. Laser Beam wrote:
^ One of the other young'uns on the bridge in TWOK was wearing an officer's uniform but with a red shirt underneath, indicating cadet status.
Correct. He and Saavik were assumed to be in "command school", an add-on course to the regular four years. I seem to recall publicity blurbs of the day.
As occurs today, some university students probably do courses concurrently, and others come back for additional courses.