My daughter did early entrance to first grade at a small private school. This is another way of saying that they tested her when she was 4 years and 9 or 10 months old (the age cutoff for entry to K at the time). She passed the end-of-K tests and a few end-of-1 tests, and they enrolled her as a first grader with subject acceleration at 5 years and 0 months when school started.
IMO, this is probably the best option in California these days. Small private schools tend to be more flexible in that regard.
I don't live in CA so I don't have any direct experience, but fwiw I have a friend with a dd who is now in high school who early-enrolled her dd the same way Val did in private school, and then transferred to public for 2nd grade.
Best wishes,
polarbear
