My son went to a first-grade classroom for reading, writing, and math instruction while he was in K. He stayed with K for science, morning activities, lunch, "specials" etc. All in all it was something like a 40/60 split (40% 1st grade). In 1st grade, he was excused from most on-grade classwork in reading and math and given a great deal of independent work, which he was able to handle as he is self-motivated and mature. I don't know if these options will work for you. Just giving examples of what we did.
