I have resisted responding on this thread but have caved in.
I am a maths teacher with a maths degree at a comp in an area with a boys and girls selective schools in the same town although it doesn't make a big impact on the cohort. Selective take 120 each per year from a huge area. We take 240 per year with Sports being our specialism.
At my school in the maths department all classrooms are effectively sat in rows or similar with everyone facing forwards. We sometimes rearrange for odd lessons but in general it's rows. In 3 schools I have taught in this seems to be the norm.
Last year I taught top set for GCSE and also set 8 out of 9 sets so pretty much bottom set.
I used my professional judgement to show my lower set certain high grade topics I felt they would be able to grasp to boost their chances of better grades. I did decide not to teach some topics as I knew they would really struggle with the concepts and it would knock their confidence hugely. A huge part of my job is building up these young people's confidence.
Since becoming a teacher I have assisted so many adults to redo GCSE in Maths as they need it for future study - the biggest issue they all seem to have is having been told they were rubbish at the subject so they have no confidence.
Mixed ability teaching is tough in maths, until we set yr 7 we teach in tutor groups. I can have a class of 25 students who range from level 6 (2 levels above end of primary expected) down to a student who is below level 2 so still has the ability of an average child in a reception class. I will admit that is a challenge to teach anything meaningful for all. I have (before I suggested changes to topics) struggled to teach a basic algebra lesson to such a wide split, one student moaning loudly it was easy whilst another sat in tears as 'he was always going to rubbish at maths'. This does nothing positive for anyone. So I do support setting in the current method of teaching.
With regard to revisiting previous taught topics a lot of Primary maths is not taught well, I know a lot of primary teachers who don't understand maths deeply enough to be able to teach 'mastery' they learn it just before they teach it - sadly this is creeping into secondary at an alarming rate as well.
So we do have to recap to iron out any previous misconceptions - but I don't assume no knowledge but as Noble says you would be shocked at how little students recall. A huge part of this problem is they are the 'google' generation where they don't need to retain information as it's all at the touch of a button - this is an attitude I have discussed with a lot of 16 year olds at length, they struggle to retain stuff as they don't often need to a skill that is slowly disappearing.
With regard to the TV programme I agree the students are being portrayed as a group of some of my worst ever classes. I think they were poorly prepared and didn't 'buy' into the experiment. Maybe an inter school challenge would have worked better. I do feel they are showing a huge lack of respect and very poor manners - but I hate to say it in my nice town and middle classed school it is very typical Yr9 behaviour I am just limited to 20-30 of them! The SLT seem to also be lacking the respect and manners I would expect as well.
Apologies for mammoth post and typos am on my phone.