Is it not a typical Gryffindor pasttime to rewrite textbooks?

Only, generally it's scratching out things and replacing them with dirty words or stealing your neighbor's and having conversations in the margins.

Still. Putting things that were not in the original = rewriting. :| We are practiced.

I'm very surprised at Ravenclaw, though. I always thought they did actual for serious corrections because the textbooks writers were ignorant. .-.