You're quite right -- doing math and writing about how you did math are not at all the same thing. You have to have a teacher who is capable of understanding the higher level of reasoning that a gifted math student uses; if the teacher doesn't understand the kid's process or the way it's presented in the writing, she probably just assumes it's wrong.
My DS writes some far-out explanations in his e-school algebra, on problems where they have to describe a process, but fortunately for him, the teacher seems to be able to follow him for the most part. Another teacher would assume he didn't know what he was doing and had just copied the answers from somewhere.
