For those of you who came here looking for the term's idiomatic meaning, that's a Cloud Cuckoo Lander.
As for actual space cadets:
A Discredited Trope from the early Pulp Magazine days of media science fiction (peaked in the 1950's in movie serials and TV shows), involving a hero who was part of an organization that handles law and order in outer space, much like in a Western... in space. Frequently the titular Space Cadet was a child or teen who was a new recruit or a sidekick to an older hero. May be the equivalent of Walking the Earth with isolated outposts and frontier planets; especially strange when the Kid Hero seems not to have a family at home. His training place would be the Space Cadet Academy.
- Commander Cody.
- Buck Rogers, especially in the comic strip and movie serial, where the kid sidekick Buddy was added.
- Dickie in The Adventures of Captain Bucky and his Space Marshals, in Outer Space.
Dickie: Gee golly, Captain — I would have never thought of that!Captain Bucky: Well Dickie, that's why you're not The Captain.
- The Last Starfighter is about an Earth teen who gets recruited by aliens for this sort of organization.
- What Jim becomes in Treasure Planet.
- Space Cadet: By ** Robert A. Heinlein novel. Notably interesting for being perhaps the Ur-Example, and showing what modern readers will easily recognize as a cell phone in the opening pages.
- His earlier story Misfit also fits the trope, and makes the inspiration from the Depression-era Civilian Conservation Corps more explicit.
- Tom Corbett, Space Cadet (based on the novel)
- The Lucky Starr series of books by Isaac Asimov, which started with David Starr, Space Ranger.
- Jeff Wells of Isaac and Janet Asimov's Norby series is also technically a space cadet, although most of his adventures are unofficial ones.
- Various midshipmen throughout David Weber's Honor Harrington stories, including Harrington herself in the novella "Ms. Midshipwoman Harrington".
- The Scott Saunders Space Adventure series by Patrick Moore.
- Space Cadet Happy from Space Patrol (1950's radio and TV)
- The Video Ranger from Captain Video and his Video Rangers
- Winky and Tagalong Kid Bobby from Rocky Jones, Space Ranger
- Tom Corbett: Space Cadet, as well as his companions Astro and Roger Manning, all cadets on a training ship. Rod Brown of the Rocket Rangers had a similar premise.
- Wesley Crusher represents the best example in the Star Trek universe. As a child prodigy, he represented an Audience Surrogate for a young viewer and could be considered a "sidekick" to Picard. Later in the series, Wesley became a literal cadet in Starfleet.
- The elite cadet group "Red Squad" featured in Star Trek: Deep Space Nine is something of a Deconstruction of the trope — especially in the episode "Valiant", which shows what happens when a group of cocky cadets get to fly their own ship without adult supervision. They attack a Dominion battleship and get blasted to hell.
- Nog becomes a cadet in Season 3 of DS9 before his Rank Up to Ensign.
- Icheb becomes a long-range version (as in 30,000 light years from Starfleet Academy) in Season 7 of Star Trek: Voyager, being instructed by the command staff of Voyager.
- The Outer Limits (1995):
- In "Quality of Mercy", Bree Tristan tells Major John Skokes that she is a cadet 2nd class who was assigned to Europa base and that she was captured by the aliens while on a training mission with her instructor Commander Hartley. In the final scene, it is revealed that Tristan is in fact an alien in disguise. It is not made clear whether Tristan was a real person whose identity the alien assumed or whether she was merely a creation of the aliens.
- In the sequel "The Light Brigade", the cadet was assigned to the Light Brigade during its mission to destroy the aliens' homeworld with a subatomic bomb due to the influence of his father, a member of the UNDF Council. He wanted to be one of the 600 heroes who would save Earth from the aliens. It doesn't go according to plan.
- Buzz Lightyear from Toy Story is an homage/parody of this trope. He later received his own spinoff Buzz Lightyear of Star Command, in which he plays the trope straight.