Understanding Comics by Scott McCloud is a textbook in the form of a graphic novel. It is an essential book for anyone teaching a class in Cartooning, Comic Illustration, or Animation. In fact, many of the subjects that McCloud speaks about in the book are great for use in other art classes as well. I can't recommend this book for art teachers enough- even if you have never mentioned comics in the classroom before. The sequel Reinventing Comics also has great information and provides a unique view on comic creation, the artistic process in general, as well as the industry and direction of the medium. Scott McCloud's website has some great games to get the creative juices flowing on his inventions page (that could be used for any idea-generation exercise), and a bunch of unique approaches to comic creation on his online comics page.

Update: Here's some very useful information on how to print comics.