We moved to Nevada (Las Vegas) when DD was in yr 2 in the UK, she has to enter school in first grade. We did push but our school district insist they remain with thier peer group for grades 1 - 3 in state schools.
She was miles and miles ahead of her class mates in everything. The only thing they quibbled with her about was spelling and they had somewhat of an inability to understand that American English and British English have such different spellings for so many words. It took a good 6 months for the teacher to understand that we really do spell things differently and that although yes she needed to know US spellings if she lives in the US, she does not have a problem if she does not know them on arrival.
DD was operating on grade 3 levels for Maths, Science, Reading, Writing etc. She made a few freinds in her grade but many of her freinds here are 2 or 3 years older than her as the maturity level seems to match. She was doing well but was not above average in the UK.
We are only here a few years, and then we will be hopefully back to Europe for a posting there before back to the UK in time for secondary school. I work with her out of school to keep her on par and ale to blend back into the UK system. We do a few things:
We have our own British English - American English Book we add to when new words that have different uses or spellings come up.
We use the UK version of Mathletics (www.mathletics.co.uk) and DD uses it at her current UK level to ensure she is keeping up with things back home.
We also use the UK version of Spellodrome which is a sister site to Mathletics. This is like an online spelling bee. It is very handy and keeps her fresh.
I went to speak to the Librarian in the school ( and go back to remind her every year) to encourage DD to read at her level, as they have a weekly library visit and are taken to an area in the school library and encouraged to bring home books from that area. DD was bringing home what she considered baby book as they were only allowed to choose from that section. Equally I spoke to the teacher to make sure that in class room reading time she would be reading chapter book rather than picture books which she was being given.
All in all it does depend very much on the school district you are in. Here in Clark County School District they stay very basic in grades 1-3 then suddenly raise the bar 3 fold and are suddenly working at a much higher level. So I think it does even out but right now it is frustrating and I feel if I don't do things to augment her school work she will just get bored and be naughty.