I am not going to counter anything noble says about maths : wouldn't dare!
But there are some very ugly things about setting. I work in a department which sets 'rigidly'. In practice, especially in the intake year, the sets are a joke. We spend a large amount of time discussing misplaced students, do little , if anything about it, field endless fretting emails from parents and ghettoise PP students and boys into small bottom sets : we NEVER discuss the pedagogy of teaching in sets - NEVER discuss how to stretch the able or support the weak. I'd be willing to bet the schools that have actively chosen a MA policy have been engaged in discussion and training.As a result of our policy of small bottom sets, even set 7 of 9 is 28 -30. Behaviour issues dominate in set 3 downwards . Girls dominate top sets. In year 10 we have single sex groups as an experiment : but they are still set. Guess what the set 4 boys are like to teach... I would say the bottom sets aren't given to the best teachers : it would be heavily resented if that were the case. in fact, over the years the - let's say favoured rather than - best staff have had set 1!
On another note, my DSs go to a school obsessed with setting in everything and , as a parent, I hate it. DS1 was placed in set 2 when he arrived there. This meant ,because he was set 2 English, he was set 2 for everything (no sense of flexible abilities , apart from maths which had its own sets), There were 33 students and 2/3 were boys. The change in his behaviour and attitude to learning was instant. DS2 who is more studious, but not brighter than, DS1 has squeezed into set 1. He is getting a much better educational experience. Why should the just below the most able suffer on the altar of the high (and allegedly low) achievers? I predict now that DS2's GCSE results will exceed DS1's.
DS1 took Spanish GCSE - was bunged in set 2 and got full marks. This could suggest he should have been in set 1- or, more likely, it suggests his aptitude for MFL was never recognised by such a rigid school.
I teach a MA GCSE option subject which has students from grade 9 to grade 2 ability. It is a haven. I could argue the weak students are maybe a bit confounded at times, but that will always be the case for them. The same boys are in my set 8 English group and are doing better in the option subject. They are very similar subjects in many ways. The grade 9 girl is absolutely soaring. In idle chit chat the other day, her English teacher hadn't even noticed she existed really and just said she was 'quiet'. She has got lost in her set of 30 high achievers and isn't being pushed other than at the same pace as the rest of the class....
Lastly, I notice newer teachers into the profession are horrified at the notion of anything other than rigid setting and say they couldn't teach anything but. So they have clearly been ideologised and institutionalised! And deskilled.
Finished rant!