One of the most memorable things about this book is its lack of a proper nineteenth chapter, which is instead only a few sentences long. Check it out:
There is no Miss Zarves. There is no nineteenth story. Sorry. (19.1)
This, of course, is due to the fact that the builder of Wayside School forgot to build the nineteenth story. But the thing about that is that it's impossible. In the real world, you can call a building's stories whatever you'd like, but it is simply not possible to have thirty stories but not a nineteenth. Because of this, one of the things that the missing nineteenth story symbolizes is the silliness of the world Sachar created in this book. Every time it comes up we are reminded that all bets are off at Wayside.
Though the nineteenth chapter—which is also the nineteenth story of the book—is almost as nonexistent as the nineteenth floor itself, something pretty cool happens in it. Let's focus on the last sentence:
No apology needed, right? The whole nineteenth story problem makes for some good laughs, after all. But none of this is what's important—what's important is who is being apologized to. And that would be us, the readers.
Pushing aside the barrier between the story and the audience by speaking to the reader is called breaking the fourth wall. And while the nineteenth chapter isn't the only place Sachar does this (check out Chapter 20 to see if you can find it happening there too), it's a really great example of breaking the fourth wall because it's such a short little chapter. With nothing after following the apology, we really notice that we're being spoken to.
So let's do a little math, shall we? Breaking the fourth wall + no nineteenth story = this is one unusual book about one extraordinary school. Yup—that's the nineteenth story as a symbol in a nutshell.