Most students are ground down by the end of their GCSEs and fancy a change too, lots leave and lots arrive at all schools, lots go for co-ed from girls schools, that can be a 'refreshing change' in itself. In my day job i see a percentage of girls leave tormead for state sixth form for boys, less pressure, a change, money, or many other reasons. I see a lot of academic kids leave state and go to Tormead too for A-levels, alongside other establishments.
For some a college environment is better, whether its academic, less pressure or vocational, kids have a fit and sometimes with the best will in the world places don't work out.
My nephew moved from a academic independent school up north to a state college in septemeber due to stress, and the pressure dropped straight away, and he's really happy.
Sometimes even traditionally supportive environments aren't the right places, just because of 'fit', and moving is the best thing to do.
On teaching, I guess thats a taste thing too, and the PPs opinion is valid and is a personal experience, so is real. That said, regardless of teacher turnover and teaching, Tormead gets some of the best results at GCSE in the whole area, so cant be too far off getting it right.
Anyway, like you said, you're done - wishing your DDs the best of luck!