What's a good % GCSE a*-c level for a High School...

[seeker]

...in a very socially diverse area where the "top" 23% are creamed off by the grammar school?

[senua]

Is it obvious, NG?
Which obvious answer are you going with - that they are all not-so-good at all subjects (so no-one who is a whizz at Geography, say, but failed to get into the Grammar because their Maths let them down; no-one talented at Art but English is their second language; no late developers) or that the teachers aren't teaching the whole syllabus (eg in Maths, you have to cover certain topics and be entered for higher tier to get the A/A* marks).

[noblegiraffe]

senua, a quarter of the kids that should have been going to that school have been taken out and placed elsewhere. Not any old quarter, the academically bright ones. The ones who would have been getting the vast majority of the A*/As. A few of the kids left got the odd A, like you said they might just have the flair for 1 subject. And the ones left also have a relatively high proportion of SEN, which, like it or not, will affect the numbers at the bottom end.

If they had loads of A grade kids who weren't getting As, it would show up in the CVA. And the Ofsted report if seeker has read it.