Simple
Sideways Stories from Wayside School is not only written for a young audience, it's written to entertain a young audience. And nothing ruins a joke like having to look up a word halfway through reading it, so Sachar keeps the language simple and straightforward to make sure everyone gets all the punch lines.
Characters are described in just a few words—"Joe had curly hair" (3.1), and "Sharie had long eyelashes" (4.1)—and the book is full of bright, funny dialogue. It's almost like Sachar is coloring with big, bold crayons when he writes, rather than using subtle oil paints or watercolors. His style fits in perfectly with an elementary school full of kids.
Sachar has also chosen to write a collection of thirty separate stories, instead of a long and twisting adventure. Why? Well it's a great way to focus on the kids in the classroom and all of their separate personalities. For a book so detached from reality, this approach seems very true to life—if you went into a classroom and had to learn about all the kids, you'd probably spend some time with each one of them, too. Plus, writing thirty little stories really suits the classroom environment, where young kids can't often—or won't—pay attention for a super long time.
Sachar creates Wayside's funny world by writing about it a lot of different ways, and the end result is a patchwork of tales that create one very vivid—and entertaining—picture.