Sounds like the high-school based “Junior” reserve officers training units that we still have in the U.S. Not active service, but trainees/cadets that haven’t yet formally enlisted.
No, these were full time Officer Cadets (Cadet Midshipman) or Junior Recruits. They were full time sailors. Lived on based, and trained every day. They even went to sea, and some went to war (although policy said they were supposed to keep you out of a war zone until you were 18, I know plenty who served on ships in Vietnam younger than that).