Re setting.
In years 10 and 11 as students chose GCSEs it's quite rare for GCSEs that are not compulsory to be set.
So the compulsory GCSEs are maths, English lit and language and science (either double or triple)
The other GCSEs people choose, and they are usually put into option blocks.
So if for example twenty people choose German, they are the twenty people in your GCSE group and it will be mixed ability.
Most schools have some kind of pathways system to encourage students to do GCSEs that they are capable of. So high attaining students can choose what they want. Students who are lower attaining may have to drop a gcse (often languages) to do extra maths and English in that time.
So if your child chooses (for example) history German Spanish and Latin at GCSE they are likely to be in classes with middle to higher attainers at least for those options.
If they choose art, child development and btec PE they are likely to be with lower attainers.
What this means in practice is that even in completely mixed ability schools at gcse there is an element of separation due to choices made.
That having been said, mixed ability in science and maths is quite controversial. English less so as everyone sits the same gcse (there is no foundation/higher split) and students can access the same material at a different level which is much less the case in maths and science.