In my experience, subjects are blocked together because of the timetable. So if you are in set 2 for Maths, you are likely to be in set 2 for English and Science, regardless of your ability in each.
We had four (max five...although that was rare) sets for each half of each year grp. Each set had a parallel set on the other side of the year.
I have to say, I don't agree with it. The kids normally spend the whole year in the sets they are given at the start, and as I hinted at earlier, those sets are pretty similar or virtually the same for each child, regardless of the subject they are in, or the ability that that child may have in a specific subject. The bottom set kids spend all day with each other to which isn't good. They become totally unmanageable because very often, the naughties are often 'dumped' in there to.
I agree, the camera would have had an impact.
That class looked pretty normal to me, although we only really saw a small handful of the 24 in the class didn't we. Apparently, the camera crew were there for ten weeks...that means that a lot of material was edited out, and therefore, not shown.
A word of caution...I am sure the school had a pretty tight reign on what was filmed and shown. They wouldn't have set themselves up for a whole load of bad press!