My HG son did all right in K but he was accelerated part-time to 1st and the teacher gave him a lot of his own work to do and excused him from some assignments. Basically, he got the K experience but was allowed to skip some of the K boredom. I don't know how common this is. His teacher was special. For reference, DS was reading Harry Potter in K. His math skills are not as strong but he was certainly beyond K math.