We moved away from England in December when DS was in year 6. So after six years and a term in the same school, mostly same classmates/friends, we left two terms before 'graduating' with his friends from primary school.
He missed out on some things that are usually done after Y6 SATS (end of primary national tests). Such as leavers' ball, school play, special assembly, special awards. 'Yearbook' type photo album, leavers' hoodies, Y6 residential trip. That did feel kind of a shame, after all that time in this school, then leaving just before the finish line.
OTOH he also avoided half a year of SATS prep - as we gathered from friends, most of that final year of primary school was 'teaching to the test' and everything was about optimising the school's exam results. Not the same at all schools, but definitely the case in this one, that Y6 was quite a wasted year, with no new learning, just tons of test preparation. We weren't sorry to have missed this!
By the time DS' mates were starting their new schools, DS was already well settled in his new school.
Over all, I don't think it was wrong to leave his primary school a little earlier than the rest of his cohort. It would have just delayed the inevitable - and he missed out on some fun stuff, sure, but also he avoided some less positive stuff.
In a new country, with school in a language he is confident in but is not his first language:
It has always been, and continues to be, completely sufficient for him to be able to do really well in all school subjects except the local language/literature. As eg written answers in a history test aren't marked down for spelling or grammar mistakes. In language/literature he can get great grades at times, but even four years later he can work as hard as he likes, he simply cannot get to the same level as the equally able (as judged by other subjects) native speakers.
He can do at least as well as not-so-able native speakers, just not as well as the 'top stream'.
Socially, it took him about a year for his words to come out at the necessary speed (before conversations moved on etc) and in that time he struggled a bit and was a bit isolated. It helped to be in 1-1 situations; in groups, things tended to move too fast for him to join in properly.
So from my perspective:
- don't worry too much about 'graduating' with the cohort at your current school
- your child's English sounds perfectly good enough to be able to keep up with school subjects
- it does take a little while to settle in with a new language, new school system, new friends, so having some time before work on important exams begins, would be valuable
- your child will most likely be at a disadvantage compared to native speakers in English language/ English literature GCSEs, but moving a year earlier or later will make little difference to this, it won't turn her into a native speaker! Nevertheless, with effort, she can get decent grades, though she may have to work harder for them.
Another thought: Certainly not all schools are like this, but I've read on MN of kids joining English secondary schools from abroad, and as they have no (English) 'record' of ability, they get put into whatever ability group that has space, or into the lowest ability group by default. This is certainly not good practice but has popped up on here every now and again as something new incomers have experienced. So your child might need some time in her new school to work her way up the ability groupings. (It is important to understand that lower ability groups are often not taught the full curriculum needed for top grades, but rather, only what is needed for a pass grade; so if you have an able child, you don't want her to be stuck in the lower ability classes).