Two answers: first, one about me, then one about my son. You asked about experiences with public schools, and I went to public schools. DS goes to a private school.
I was allowed to work independently on reading, math, and spelling in 3rd grade. I just went to the teacher when I had a question. This worked out reasonably well for the most part. I had to repeat some Language Arts stuff the next year, even though I'd tested out of it, though. You might try asking them about this approach. Just make sure your son won't be repeating stuff next year.
When I was in fourth grade, it became apparent that language arts class was too easy for a small group of kids in my class. They found someone who spoke Spanish and pulled us out of LA and we did Spanish instead. Everyone was a lot happier: we weren't acting out in class and we were learning something fun and useful. So you could also suggest letting him do something completely different like a language or music or whatever.
My DS9 has had two skips (2->4 then 4->6 this year). He goes to an extremely small private school that understands gifted kids. Overall, the work seems to be 7th grade level.
He's also doing algebra. He had done a lot of algebra with me already and we decided to start him at the beginning to give him easy math while he gets used to the new grade level. His work at school is finally meeting his needs. It's great --- really great.
Anyway, he also had subject acceleration in math last year. It worked very well for him. He just sat with the sixth graders during math class. No special accommodation was needed because he was just sitting with the sixth grade kids. It was easy-peasy.
Just wondering: is there a K-5 or 6 school that he could attend instead of his current one? To be fair to the school, it'll be hard for them to meet his needs given that they're not set up for older kids.
Val