Trafford.
Getting into on of the grammar schools is significantly harder than it was a few years ago. It's got more competitive and more kids are tutored. This has been amplified with Trafford being singled out by Hong Kong property brokers as one of the best places in country to live for affordability, quality of life and education standards. This has brought Chinese style hothoused kids into the frame. There's also been a boom in kids for the years currently going through High School (the birth rate drops off a cliff in a year or two).
The pushy parents who don't get their kids into grammar school because they arent academic enough, send them to private school if they can to avoid the comprehensives which don't have the best reputations.
So the top tier of kids go to grammar schools and the rich but not so top tier kids go to private schools.
The grammar schools in Trafford have extraordinarily low rates of SEN kids.
Conversely there's also a lot of better off parents with kids with SEN who sent them private because of the better staff ratios.
I imagine the pattern is replicated in any area with grammar schools/affluent area.
It therefore shouldn't be a surprise to see this pattern.
In high birth years, you would expect the pass rate to go up, because the admission criteria will be higher for that year group due to additional competition and demand. You would expect the opposite to happen when the crashes birthrate hits high school. Trafford is so expensive, there is a massive drop in pupils, so the standards are likely to slip with more children bussing in from further afield.