Claude only exists in the mind of Germain, or vice versa?
I watched the movie yesterday with my girlfriend and she interpreted the plot as the fantasy of a teacher who recently was left by his wife and lost his job (maybe because he ridiculed a pupil in class?). According to this theory Germain is a bitter teacher and failed author who dislikes himself and despises everyone else. What we see in the movie is what is going on in the mind of Germain as he tries to process the trauma of losing wife and work by making a story. Claude might represent Germain as a teenager or the son he never had? The Artole family maybe represents his own boring middle class life?
I had a different theory: Germain is a character in Claude's story. Claude is a lonely teenager who longs for a father figure. He fantasizes about his French teacher adopting him as his protege. This troubled kid is both envious and contemptuous of the Artole family and wants not only approval from them and Germain but also to control them and prove his superiority. The story of the movie is being written by Claude as we see it, very much like in "Adoptation". Pretty much nothing is real, although the characters are taken from Claude's real life.
Eitherway, lots of things suggests that Claude and Germain might be just one person, for instance the simularity of the title of Germain's book and some sentences in Claude's letters. Maybe they are each others alter egos? (Don't ask me how that would work ;))