Is it by ability or by developmental readiness ?
Subtle but important difference.
The system is slightly different in Scotland (and handled better by ds' school) but I'll tell the story of how it happens at ds' school (using him as the example).
He's in theory slap bang in the middle of the age group, but in pratice ('cos in Scotland you can genuinely defer) he is at the younger end, pluss he was quite young/innocent character wise.
His school had a "straight" P1 (= Reception) and a composite P1/P2 class. He was put initially into the the composite class but all the parents were told that the children would be assessed and there would be a re-shuffle at the September weekend (it after 6 weeks, when they had only been doing half days). He was changed to the P1 class because that was the peer group that they felt he would be happiest with - which incidentally included one of the oldest kids (ie one who was nearly a year older than him, having been deferred, as we can do in Scotland).
He coped fine with that and ended up in the top groups for Language and Numbers. The Language one was misleading as he was learning everything off by hear and not actually reading - because he wasn't actually developmentally ready for reading even though school gave him lots of extra support but said that some kids only "get" it age 6 (and he was over 6.5 before he got it).
Cut a long story short, he has now finished P6 and is an excellent reader and doing well with his numbers. He's probably amongst the brightest in the whole P6 cohort. Over time, the developmental differences even out.
The kids that were in the composite class this last year ended up being composite P6/5 (ie down a year rather than up a year) because the P7 class could be squeezed into a single classroom.
To be fair on the school, the groupings for maths and languages tend to be across the classes in a child-centric approach. So for example, ds' maths group (the "top" P6 group) did their Maths with some of the P7 ( = Y6) kids.
I suppose that is long-winded way of saying "Don't sweath it" - and just be grateful that your ds has a good teacher next year! In the end, that's all that really matters not what mad ladies are saying on fb :)