Clip Art, Backgrounds, and Themes are broken down by whether they should be shown to elementary schools, high schools, or both. In order to use this, you need to set the type of your Book's project. This can be done either when creating the Book or later under Project Settings.
When you are first creating a project, you will see an option on the last page to set the Project Type.
If you forget to set the project type when creating the Book, or maybe just set it to the wrong type, you can change it at any time from the Project Settings.