Flat Stanley is such a fun character to add to any classroom. I would use this book and others in the series in 2nd grade or 3rd. Many classes have made their own Stanley's and mailed them to friends or family in far off places. When the Stanley's return, it is a great individual project to write about or do an oral report on where Stanley went and teach classmates about that state! Often, participants send back photos of Stanley and his adventures.
Including this book in a social studies curriculum would be an excellent idea so that literacy is spread to more than just language arts class!
grade level equivalent 4.4