I had an interesting talk with the parents of a PG kid this weekend. Their son will graduate next year at 19 from college.
Their son went to a Tier 2 private school in the DFW area and graduated when he was 17. The school kept him with his age peer classmates for all but math and science when he came in in the 5th grade and had pull outs for classes - doing calculus his 9th grade year and then did upper division college math with the head of the school who was also a tenured math professor at a local college. The same professor also took him through quantum in physics and he then took some college classes on his own in Chemistry. He started college as a junior.
If they "had to do it all over again" and wanted to stay in public, then they would find a sympathetic private to accept him into 1st as a 5 year old and then get him pullouts, then put him back into a Tier 1 public high school after he was working across the board at a 9th grade level. They said that the public schools would take a kid who finished any of the middle grades in a good private with high grades.
They said having K-12 under the same roof at a private made the pullouts work.