You know how some grown-ups talk to kids like they don't understand anything? Yeah, you know the type: they assume that just because you're young, you don't know anything about the world. Well, luckily for readers of The Phantom Tollbooth, Norton Juster is not this kind of grown-up.
Our narrator sounds like he's talking to a total equal. He brings up complicated concepts, like idiomatic language and infinity, and treats readers like they can understand it all without any problems (sometimes he might even be expecting too much from us!).
In the same way, the book never gets all moralistic or makes us think that if we don't enjoy school, there must be something wrong with us. The Phantom Tollbooth presents learning as a process of discovery. It's exciting, and it's something to be celebrated. Having an open mind leads to more adventure, and the rewards are enjoyment and adventure.
Bottom line: our narrator (and hence, our author who created him) respects us. And it totally comes through in the matter-of-fact, mature tone of the story.