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.
Juster frequently plays upon the dual meanings of words and expressions to create humorous situations. When Milo orders a "light meal" at Azaz's banquet, he is literally served light. When Milo catches a word on the tip of his tongue in the Soundkeeper's fortress, a word physically appears in his mouth. When the Humbug jumps to the conclusion that nothing more can go wrong with the group's journey, he leaps out of the car and lands on the island of Conclusions. Juster uses puns both to amuse and educate, as these unusual situations often result in Milo learning an important lesson.
Once he has learned what a given character has to teach him, Milo often receives some sort of gift from them. Considering the importance Juster places on education throughout the book, these interactions are likely meant to suggest that knowledge itself is a gift. It is also significant that the gifts that Milo receives embody the lesson itself and are handy when Milo needs to put his knowledge to work.
As their names, which play off a well-known idiom, imply, the two imprisoned princesses represent wisdom and common sense. The fact that they were found orphaned in the King of Wisdom's garden suggests that they literally were the fruit of wisdom itself. The princesses, famous for their ability to settle disputes fairly, provide an important counterpart to the knowledge and intelligence of Azaz and the Mathemagician. Milo's quest to return them is, therefore, both a quest to gain wisdom himself (through the lessons he learns during his journey) and to return wisdom to the land (by returning the princesses).
Each gift represents the lesson Milo learned from the character who gave it. During his travails in the Mountains of Ignorance, Milo is able to use these gifts as a way of implementing the knowledge he has gained. Alec Bings, for example, teaches Milo about perspective and gives him a telescope. When Milo is frightened by what he cannot see, he uses the telescope to change perspective and conquer his fear.
In addition to being a vital character who contributes to the storyline, Tock also plays an important symbolic role: he represents the wise use of time. Tock is the only teacher-character in the Lands Beyond who does not give Milo a gift after teaching him something, because Tock's company itself is the gift. He constantly reminds Milo to make the most of his time, helping him to defeat the boredom that defined his life before coming to the Lands Beyond.