At 5.5 (also with Dec birthday and Sept cut-off) my son did both K and 1st in one year. He was 100% ready for 1st grade but was not allowed a skip of K due to district age policies with a no early entrance policy. There was a strict MUST be 5 by September to attend K and MUST be 6 by September to attend 1st grade. NO exceptions allowed.
He attended half day K in the morning at public school and then he attended 1st grade for the rest of the afternoon at the same school. K was his recorded grade. The following year he skipped directly to 2nd grade at 6.5 years old. Worked perfectly for us.
At 8.5 my DYS is now going into 4th grade at public school and taking Middle School Math and Computer Literacy courses through our states Virtual Learning Program as a part time student.
Repeating a grade I don't think would be a good option for a HG+ child unless the program and curriculum would be very different.
