There are intestine points about research in the area, however I do feel that the implications of this need to be individualised. If research shows that most children do best taught within their year group, that doesn't mean it is the case for all children.
Personally I found setting at school to be extremely beneficial, and found mixed ability classes to be very frustrating. Perhaps mixed ability class teaching can work well in certain circumstances, but there are many hurdles to overcome.
In Dd's school there is a very wide range ofabilities, as well as children from very difficult backgrounds with challenging behaviour. I understand this must be difficult for the teacher, who I do believe is good.
Interestingly, I was told prior to the year starting that children would be put into sets for certain areas of teaching, but this never happened, and instead there are mixed ability groups. I don't feel that this works for dd, as she is complaining of being bored and the work being very easy. Looking at the level of work that she is doing at school compared to home I agree with her.
I'm not sure how to best manage this. I am offering her 'extension ' work at home at her request, but the result of this is also her becoming further ahead of her peers, and becoming increasingly bored at school.