I think it’s the case that any school which has its own Prep that feeds through into the senior part, will have a wider range of ability at Senior level than one which selects everyone from an 11+ exam.
I know that schools ‘manage out’ some of their weaker students before senior school, but there are always some that scrape into the seniors who wouldn’t have received offers through 11+. Not to say that this is necessarily a bad thing, and as others say, the fact they can squeeze (and it really is a squeeze in some cases) the amazing results they get out of some of them is a huge achievement on the part of the school.
And in the end the level of selectivity is a numbers game. Being outside London will mean applications per place will be a bit lower and so the school has less to choose from....still plenty of choice but not at the level of a London school or a state Grammar. They are bound to have some at the ‘bottom end’ who wouldn’t have made it into the schools with a bigger applicant pool.
And there’s the issue of consistently being at the top of the league tables....in some ways the only way is down. No school wants to drop and so they have to devise more and more ways to remain at the top. You can either be more and more draconian in managing out earlier and earlier, Or limiting the range of choices available to certain girls and only letting them go into the smaller range of subjects where top grades are possible for them, or you can improve teaching etc to a point, or you can increase the pressure on staff and students to achieve more. Even when some schools try to avoid adding to pressure, they still do it because stress and pressure just abounds even when unspoken in some schools. It can filter down from management to the next level who feel under pressure, who then pressure the heads of subjects who then pressure the teachers who then pressure the students.
GHS fees are certainly at the lower end for a school achieving at their level, compared to the London schools and their applicants per place is likely to be lower than London schools purely because of location, so what they achieve in terms of results is pretty amazing really. But it won’t be and can’t be easily achieved and that’s not without cost somewhere along the line.