What causes Finny to break free of these fantasies?

Mr. Ludsbury, one of the teachers, discovers the boys at their morning exercise; he tells them to keep it up as some kind of preparation for the war, and Finny just blows up at him, telling him it has nothing to do with that. Mr. Ludsbury turns red, gets angry, and stomps off; Finny has no remorse for angering the man, though Gene feels somewhat strange about what happened.