The imaginary Ferris is real to his imaginary sister and parents, and in all the imaginary interactions with school administration.

If Cameron, too, isn't real, there is no movie: a dream requires a dreamer. The whole movie, i.e. Cameron and his dream is, of course, John Hughes's dream, and then the audience's.

  If we shadows have offended,
  Think but this, and all is mended—
  That you have but slumbered here
  While these visions did appear.