In our district, you must register for age grade no matter what you were doing in homeschool. And then the principal might move you from there.
Our school district's policy says the same thing. However, I have it anecdotally from another parent that they homeschooled a year ahead, then registered their child at the new grade level for public school the following year, and the school accepted her child at the grade they said.
This is not the first time this district has failed to make policy and reality look alike. It's just the first time it erred in our favor.
We are in the process of doing this very thing. We pulled DD out of 2nd grade, identified/addressed all the gaps the school had created, and then began teaching her through a 3rd grade level. She has taken the statewide assessment test given to 3rd graders, which we intend to use as ammunition should she decide she wants to give school another shot at 4th grade next year.