Moving children up and down a set also is dependent on numbers, unfortunately. If you have an intake of 30 children per class or more, as was often the case in a High school where I was head of faculty- it can be a nightmare. Because the only way we could have slightly smaller groups for remedial/special needs, was to have classes of 34 or more in the top sets. We didn't even have space to sit them at desks! So if a few children need to be moved up, some would just have to be moved down.
If comprehensive education was properly funded and not creamed off- a lot of those difficulties would disappear.
Re more children doing better at GCSE, I can assure you that most schools spent a huge amount of time and energy at spotting children with potential who, for some reason, be it behavioural, social, or whatever- were not achieving it- and then spend more huge amounts of time and energy finding ways of helping them by any means possible, instead of letting them fall through the net. I am particularly proud of having been a small cog in these developments, and we achieved amazing results with many of them.