Not selective like Eton/Westminster same CE pass mark for years I believe, 55 average.
But the school has, from speaking to people who were there before me (say before 2000/2005) compared to my relatively recent time (left under 10 years ago), really improved. Including Academics. But I think, rather, it is down to an ethos change, it is quite hard-working now. There were genuinely people arriving in bottom sets that ended up going to Oxford and I remember working v. hard and being challenged to be driven.
But even then, there were still a few people that didn't pull their finger out and didn't reach their potential I guess. Same as any other school.