Thanks, @Iamnotthe1. I confess I don't entirely understand the low progress scores point. I will reread the guidance.
I had assumed that the number of entries at the school would have been too high to have low numbers of students entering to allow 24% of them to get top grades, but I've just checked the latest Ofsted report (Jan 2020, based on a visit in November 2019) and the size of the 6th form was only 170. Assuming that the numbers are split equally between the two years, that's only 85 students entering and I guess numbers in many subjects could be under the magic 15.
It does seem hard to believe that the results at the school at justified, and given that top grades are up nationally in previous years, yet so many children have been disappointed, I can only wonder how many students have been given much higher grades that they might have been expected to get under normal conditions.
Just as an aside, I see lots of people referring to small class sizes, but isn't it about a low number of entries rather than small class sizes? That benefits small schools, and students doing less popular subjects, rather than independent ones per se (obviously there may be a link as independent schools might be more likely to offer a subject with few students).
Finally, I noticed last night that Worcester College Oxford has decided to honour all the offers it made to students, irrespective of the grades they were awarded. That sounded like a really positive move. Obviously all those holding Oxbridge offers will have been interviewed, so there will be greater confidence that the students are deserving of a place, even if they received grades lower than their offer.
Presumably most universities cannot follow suit as they won't have room. No idea what assumptions universities usually make about the numbers who will fail to meet their offers, but I imagine that they always over-offer.