Agree with epoh's response. We set up a meeting with the teacher a few weeks into the school year, and provided a list of what he was doing in math. We had compared it to the math curricula in higher grades so we knew ahead of time we didn't have parent goggles on. We were specifically asking for math acceleration, so it was also a request for her to be on our team to help get him the challenge he needed in math.
I think it's helpful if you have a specific idea of what you want out of the conversation - is it truly just a heads up for the teacher, or would you like to get something out of it? Could be as small as reassurance that there are books in the classroom at his level, something bigger like understanding how they work with more advanced kids or asking can he be excused from worksheets learning letters or numbers, to something as big as requests for subject acceleration.
