Been there... For my son K was only half day. What I did in this situation was decided not to sweat it since DS was learning other softer skills such as social skills & EF skills. As long as the class is play based and he doesn't seem bored I wouldn't worry too much this year & just after school him more challenging material if you can.
DS (kid who has tested HG+) had a lovely time in half-day K, even though he was on the older side...but I think it was mostly because he loved the new routine and being in class with kids his age. Even though it was apparent to him that he had already learned most everything academic they were teaching him by the first month, he enjoyed himself. Though there seemed to be near-universal agreement that the novelty would have worn off if we didn't make some pretty major changes for the following year.
