I'd second Grinity's suggestion of trying to observe a K class or at least get an idea of what the daily routine looks like. Honestly, a good K class should not include a lot of worksheets. Hands on, manipulatives, singing, games, etc. are the types of things I remember from my girls' K experience and it wasn't too painful even if they knew the material b/c it wasn't drill and worksheets.
Yes, there were books they read that came home in ziploc baggies and which were leveled early readers, but they went up to probably a second or early third grade reading level and again weren't painful b/c they were short and the kids didn't have to read them repeatedly.
I'd really hope to find a K classroom where it is more play than drill. If that isn't the case, I'd hope the get him out of the drill on the stuff he already knows. However, I probably wouldn't attempt to do that by having my first interaction with the school and teachers be a letter in which I express a list of concerns for my child. I'd be afraid that they'd blow you off as a neurotic parent or one who is making a mountain out of a molehill. I don't think that's the case, but I would want to make sure that they don't interpret it that way.
Could you ask the preschool teachers who have expressed these concerns to contact the K teacher rather than you? It might hold more weight coming from another teacher, even if it isn't a certified teacher.
