Many schools/communities have enjoyed reading Rules by Cynthia Lord as their one book. Rules has a lot of good topics to talk about and kids like its humor. Newbery Honor Award winner. For grades 3-7. Paperback ISBN 0439443830
If I'm understanding the concept correctly, you are looking for one book that everyone in grades K-12 can read. That being the case, I think you might need one that has several different versions as appropriate for the different grades. Try Greg Mortenson's "Three Cups of Tea." That's the one that comes to mind immediately. There are at least three different versions of it out there--the original, a young adult version (say ages 9-12) and then a book called "Listen to the Wind," which is for grades K-4.
