Guess what? It's not just about content. (Obviously that's important, but content alone won't do it.)
Most teachers at the elementary level choose books that complement their curriculum. Fiction or non, the books need to dovetail in some way with the concepts that are being taught. And if the books connect different areas? Even better. (Think multicultural issues and geography, for example.)
How do you know whether your book will complement the curriculum? Do a google search for "state curriculum standards." Start with your state, then check out others. How do they compare? Can you make a case in your query/cover letter that your book could be used by schools in a number of states?
How books are written is also important: books that assume background knowledge kids don't have will NOT be used in the classroom. Background needs to be built into the text, so the readers can understand without having to fetch a dictionary.
Illustrations? They're terrific! But they need to appear on the same page spread as the relevant text. If the reader has to flip a page (or two) to find the illustrations, teachers will skip the book. Make the information easily accessible to your readers. Ask to see page proofs and make it clear that you want a layout that enhances readability and retention of information.
Teachers and librarians also need to teach study skills. How? By choosing books that have a table of contents, a glossary, an index, and a list of additional resources. List of possible discussion questions? Bring 'em on! Anything to make a teacher's life easier will be greatly appreciated.
If your book would work well in the classroom, be sure to mention how it addresses these kinds of things in your query letter. Many schools use the SQ3R reading method. Describe how your book can be used in this way. Not only will you impress the editor reading your submission, you vastly increase the chances of getting your books into schools and libraries.
What is your favorite book that was/is used in the classroom? Why?