We have faced this in kindergarten, first grade, and somewhat in second grade with my younger son.
I really don't think kindergarten or even alot of first grade is academic. It's mainly social- raise your hand to go to the bathroom, sit quietly in the circle, etc.
For us, I have my kids do a little (20 minutes or so) of workbooks after school, plus reading. It's easy to do and they are both probably 2 grade levels ahead. My younger one never seems to notice or care that he "learns" things he already knows!
Our gifted program starts in 4th grade, so we are just waiting for that for him, although now they are talking about changing it to differentiation only in the classroom.
My son could read chapter books in kindergarten- the teacher, who was super nice and experienced, would smile and never really seemed to care. She would keep sending home books like Hop on Pop that were way beneath him, we wouldn't read them, etc.
