We decided not to tell the K teacher anything when he started school. We'd had no testing (or any reason to think to do it), but we knew he could read, do math, etc. However, we weren't sure what *he* would feel comfortable showing the teacher, so we just let it go for a bit. As others have said, I believe alot depends on the teacher. DSs teacher contacted us after a month or so to ask if we were aware that DS could read... That let us know that DS was not shy about revealing what he could do and so we let her know the other things we'd seen - and she clued us in to how unusual they were given his age. She initiated subject acceleration and GT testing and we were on our way.
It all worked out for us, but it really was dependent on the teacher and how she handled what she saw. Had she never contacted us and DS continued with A is for apple type stuff, we most likely would have put a bug in her ear. Good luck!
