I read an analogy once that has worked well for us though not exactly the same situation. I have actually used it for DS's current teacher and she had a bit of a lightbulb moment so maybe it would work. I told her "Imagine every morning, before you could read the newspaper you had to prove again that you know how to read. You have to sit down and read the entire BOB book collection. Only then can you open your newspaper, would you still want to read it?"
I also said once, unsuccessfully, "Would you refuse to let Steven Hawking write a book because he can't type it with his hands?" Evil teacher said "Your child is no Steven Hawking."
oops...
