Have you checked out the articles database on the Davidson website (working with schools or advocacy areas, I think)? There are some good articles that were helpful for us, in fact, I brought some with me and hit the highlights during a meeting last year at school.
Words that we have used that seem to have conveyed the situation well enough:
Asynchronous learner (used this to get around the "but he doesn't behave" sorts of objections as well as to describe how far ahead of age peers he is)
We shared testing results and put them into context (i.e., how many kids in x,000)
Exceptionally good memory, requires very little repetition (but still does require some instruction)
High ability learner
