I had exactly this question this year for DS (6). Was very surprised that headmistress asked me into her office for a chat and explained that DS's form teacher recommended that DS be put in the Yr 2 form with the older reception kids (the ones who skip yr 1) and the younger kids in his year as even though his age would have him in the older form, they thought he'd do better being one of the abler in that class than less able in the other.
At this age, i didn't even have to think about it - just said that seemed eminently sensible, as my priority for him now is that he enjoys school, has a positive attitude to learning and perceives school work as something he can do and is good at rather than as something he can't do and has no relevance to him.
Headmistress was visibly relieved -she obviously has at least a couple of unpleasant interviews every year with pushy parents who don't agree with their judgements.