LaQueen - my experience of mixed ability groups in Primary schools is not that they bring the lower achieving pupils up to a higher level, but that they leave the lower achieving pupils not understanding a thing, and leaving them to drift and fall even further behind.
It's exactly WHY I prefer setted groups in the majority of lessons.
It doesn't benefit either end of the ability spectrum to be taught mixed ability, and anyone that kids themselves that the children don't know what set they are because they've named them squares, or green, or eagles in the case of DS2's bottom set group, is talking bolleaux IMO.
My DC's ALL know that everybody is good at something, but nobody can be good at everything.
And that you put the same amount of effort into things whether you are the best or worst in the class at it, because it is the only way to improve, even if you will never be as good at Art as Jane, or as good at Maths as Johnny, you can still do YOUR best, take pride in the things that YOU are good at, be happy for others for the things THEY are good at, and work hard even in things that YOU are not good at, so you can truthfully say that YOU have done YOUR best.
No two people have the same skill set. DD can cook like a Michelin chef. She can't grasp Shakespeare for love nor money.
DS1 can understand Maths concepts that baffle my Dbro that got an 'A*' at A-Level in Maths. But ask him to draw a recognisable picture and he is unable to.
Different skill sets, that's all. Everybody is good at something, nobody is good at everything.