however it was Filmation's Norm Prescott that broached the "Star Trek Cadets" TV show..When Roddenberry refused to give up control of his baby to Paramount it was morphed into what the show actually became.
the concept later was used in what became "Space Academy".
My understanding is that the cadet/teen protege idea was for a potential animated series that was being developed in the late '60s to run alongside TOS, serving as a complement to it. It was scuttled when TOS was cancelled. After that, with no live-action show to complement, they just did an animated series that was a direct continuation.