Louis Sachar has created a strange little world at Wayside School, and the same holds true for his narrative technique. He starts the book's introduction by addressing the reader directly—"There is something you ought to know so you don't get confused" (I.1)—which is called writing in the second person. As you might have guessed, the second person is a really unusual technique to use, and writers almost never do it.
Luckily for us, after the opening most of the book is written in third person (which is way more commonly used), with occasional glimpses into the thoughts and motivations of the kids featured in each story. However, Sachar sometimes switches back into second person, pulling the readers into the story as if we are students just like the kids at Wayside. This happens most memorably in the last chapter, when the narrator reveals that Louis is the person who has been writing the book all along. Fourth wall, you're history.