This book is written for a young audience and could easily be enjoyed by elementary school kids. (In our experience, it's definitely enjoyed by elementary school kids.) However, sometimes Sachar's sense of humor can get pretty dark and weird, which is why Sideways Stories is also a good fit for the young adult lit genre. It's easy reading, but it's smart and funny enough that it could appeal to almost any age.
Since Wayside School doesn't always conform to the laws of physics, you might say this book could also fall into the magical realism category. However, there's no rhyme or reason to the universe here, so put away your serious spellbooks—anything can happen, but it almost never makes sense. Magic isn't a factor in every chapter, but since teachers can turn people into apples and invent kid-flavored ice cream, this book definitely hangs out in a world of its own.