Hmm. Good question.
I suppose I'd start off with something like "We'd like to work with you to find the most appropriate learning environment for my son."
You might want to provide some basic information about skills he's mastered, what he's doing outside of school, and where he needs work. You could also suggest that the teacher give him a couple end-of-year tests in things he's mastered. You might want to make suggestions here; stick to stuff (initially) that you know he'll do well in ("What do you think about starting with...?")
I say this because teachers probably hear from some or many parents that their kids can skip stuff, when the kids really can't. If you let your son prove --- through her measures --- that he's ready for something more advanced, she'll probably be more inclined to believe you if you come back and say, "And what about...?"
Then have a talk or two with your son about the importance of taking the tests seriously.
Before this school year started, my son's school hired a new math teacher. I told him (gently but matter-of-factly) that she'd probably give him a placement exam on Day 1, and that he needed to prove to her that he could do the stuff she was going to test him on. I explained why the test was important ("You don't want to repeat what you already know."). He surpassed my expectations and really impressed her.
I chatted (even more gently) with my then-four-year-old when they tested her to skip K and part of first, and it worked then too.
HTH,
Val
