I'll have to disagree with you on that one. And this is something Heinlein did more than once, by the way... Beyond This Horizon being a good example. It seemed to be one type of story until about two-thirds of the way through, when the protagonist suffers an attack from enemies, apparently dies and is revived ... at which point the novel veers from a fairly straightforward sf adventure-story-cum-social-commentary into being largely focused on what seems to be totally unrelated issues -- and the protagonist's character seems to have completely changed! But a second, more careful reading, will show that Heinlein planted the seeds of that early, early on in the novel, and it is that brief, one- or two-paragraph transition when he is revived, that is the linchpin of the whole thing. (This, by the way, is also an important link to his Stranger in a Strange Land, as Norman Spinrad pointed out some years ago.)
No, with Tunnel in the Sky, Heinlein is using the adventure story format to introduce the main themes of the book: the establishment and running of a society, looking at problems with various types of government (including democracy and ochlocracy), and what sort of place the individual has within any given society, and how each affects the other. There are a lot of other things going on in there, and this is one Heinlein's meatier juvenile novels (in the sense it was written for teens -- but then, Heinlein strongly felt both pre-teens and teens could handle a lot tougher meat than is generally given to them, especially literarily speaking), but those are some of the major concerns, and it's actually a very carefully structured novel, as most of his were.
Also, to put things in context, this is one in a series of novels where he examined these themes, along with others, with a growing complexity, from the rather primitive Rocket Ship Galileo (which even he felt he hadn't done quite right; it was his first attempt at this sort of thing) through Starship Troopers (which the publisher refused to issue as part of the series, feeling it was "too adult"). I've got a list of the things in order elsewhere on here, and I'll try to look it up and post it here, should you be interested. But this one comes somewhere around the middle, so he's already well on his way to exploring these ideas in increasingly complex fashion. I'd suggest another reading later on... and I think you'll find that the structure is much better than at first appears.
EDIT: Okay, here's the list:
Rocket Ship Galileo (1947)
Space Cadet (1948)
Red Planet (1949)
Farmer in the Sky (aka Satellite Scout) (1950)
Between Planets (aka Planets in Combat) (1951)
The Rolling Stones (aka Tramp Space Ship) (1952)
Starman Jones (1953)
The Star Beast (aka Star Lummox) (1954)
Tunnel in the Sky (1955)
Time for the Stars (1956)
Citizen of the Galaxy (1957)
Have Spacesuit -- Will Travel (1958)
Starship Troopers (1959)
One can include Podkayne of Mars, but neither Heinlein nor any commentator on his work that I've read consider this to be a part of this series, though it has some similarities.