Mixed ability teaching is a red herring. in know of no comprehensives - and actually no grammars either - where children are not organised into sets according to their ability for some or all subjects.
Well my comp didn't do any setting until year 9, before then everyone was taught in their forms. In year 9 it was just Maths and English. For years 10 onwards, it depending on the choice blocks - some blocks only had one class for a particular subject so there was no setting - it was purely mixed ability, in my case that happened for German and History. For chemistry there were three classes in the block, top, middle and bottom - the top did GCE, bottom did CSE and middle, which I was in, half did GCE and half did CSE - what a mess that was as the teacher was basically telling half the kids to ignore the lesson and complete a worksheet instead!
At my son's grammar, they don't set in anything at all except Maths and English for GCSE. They are taught in their forms for years 7 to 9 and then for years 10 and 11 for Maths and English there is a top set, a bottom set, but the middle sets are all mixed - last year was the first time they did this in English - before then English was fully mixed too. All the sciences, humanities, languages, arts etc are purely mixed ability right through to GCSE.