As a children's and teen librarian I love helping kids find books for school. We get lots of requests, and the specifics of each one make them like little puzzles to solve.
Say, for instance, that a 3rd grade girl needs a book that has an Accelerated Reader (AR) test and is listed as a 6th grade reading level. When elementary school kids are 'star readers' it's sometimes hard to find a book that is challenging enough but also appropriate in content. In these cases, one of my first suggestions is to take a look at nonfiction. There are so many great nonfiction books out there about all sorts of fun stuff! The specific vocabulary they use and the more complex ideas make them a fantastic choice for kids who are reading at high levels.
Here are some examples.