Perhaps check with the front office regarding standard procedures first. For our K, the teachers spend a couple of days meeting each parent/child pair and to be some basic testing. That would be a better time to bring up concerns. However, I think the best time is after the teacher has had a chance to interact with the child, perhaps a few weeks into the school year?
For our 1st thru 3rd grades, the teacher always sent home a one to two page questionnaire where you can share strengths, weaknesses, concerns and requests. However, given how crazy busy they are during the first week, sometimes the papers don't come home until the second week.
YMMV but I think there is a risk of overstating a child's abilities and leaving the wrong initial impression given that most children likely won't perform as well under the stress, pressure and distractions of a typical classroom versus home one-on-one with the parent. You can secure the best advocate if the teacher can conclude the child's ability without the parent's help. If it is the type of teacher who is hostile to high abilities, then it won't help either way.
