Though Milo never actually finds himself in the classroom, The Phantom Tollbooth is primarily a book written in praise of education. The plot arc of a bored little boy who is inspired by travels in the City of Words, the Forest of Sight, the Valley of Sound, and the City of Numbers seems to spell this out clearly. The most consistently pressed concept in the book is, without a doubt, the importance of the various lessons Milo learns through his journey. It is only by using the knowledge he has gained that Milo is able to fight his way past the demons who inhabit the Mountains of Ignorance. The princesses Rhyme and Reason, who represent wisdom, another major theme, acknowledge the importance not only of what Milo has learned, but also of how he as learned to use it.
Milo's boredom is largely the result of his inability to appreciate the world around him. His bedroom is the perfect example of this: it is practically overflowing with toys, all of which Milo finds totally unengaging. Through his travels in the Lands Beyond, he meets a number of people who also have taken the things in their life for granted. The inhabitants of the city of Reality, for one, begin hurrying about without stopping to appreciate the beauty of their city. As a result, the city slowly crumbled away into nothing. Similarly, the residents of the Valley of Sound become so unappreciative of beautiful sounds that the Soundkeeper was compelled to impose utter silence on the whole of the valley. Through his travels, Milo learns the folly of taking things for granted so much so that he decides to postpone any further travels in the Lands Beyond in favor of enjoying the things in his bedroom.
One of the defining characteristics of the Lands Beyond is the presence of quite a lot of nonsense. Inhabitants of this fantasyland engage in all sorts of ridiculous behavior, most of which shocks even Milo. Juster uses the nonsense of certain situations, such as the Royal Banquet, for great comic effect while simultaneously underscoring the lack of a natural order. Rhyme and Reason, we eventually learn, are imprisoned in a faraway castle and much of the nonsense Milo observes has only sprung up since their departure. Milo himself, of course, is in need of some common sense. His quest to find Rhyme and Reason therefore is both literal and figurative. On the one hand, he must learn all sorts of lessons in order to truly appreciate common sense. On the other hand, he must physically journey to the Castle in the Air to release the princesses.
At first Milo, simply, is bored. His tendency to be consistently bored seems to change when he first enters the Lands Beyond, but once Milo finds himself in the Doldrums he is right back where he began. Thankfully, Tock enters the scene and helps teach Milo about the value of time and how to make the most of every minute. Then Milo finds himself in the clutches of boredom at later points in the story, such as when he is in the Dictionopolis prison or when he is waylaid by the Terrible Trivium. With the help of his friend Tock and the lessons he has learned through his journeys, however, Milo manages to overcome boredom and eventually becomes so good at inspiring himself that he no longer needs the flash and excitement of the Lands Beyond to hold his attention.