Quote Originally Posted by Kindablue View Post
No, it could also be a reference to the book. But that's it.
I think it's more likely that it's not a pun at all, but rather that Twilight needs to be reminded of a "Lesson Zero," that is, the lesson that forms the foundation of all the other lessons, even if you don't realize it as you start lesson one... similar to the Zeroth Law of Robotics.