My twin DC are Y11. They go to a great school in a dodgy area (where we live), low attainment, mostly white w/c. School does have a small sixth form. Teaching is good generally I would say, but it has the staffing challenges most schools have. We know the teachers now so we know whether the subjects they want to do have decent teaching (if you move you probably have no idea unless you know people at the school), although DC not decided on all 3. Many of the talented kids leave after GCSE to go to other local schools with bigger sixth forms and better A level grades, many leave to go to college. It's a small sixth form and no one really comes in from outside the school as there's an outstanding sixth form up the road (which I don't rate that highly but they might consider).
The school has done a great deal for my kids and I'm very grateful. I feel it would be disloyal to move them! They have been encouraged and celebrated in a way they may not have been in another school. DD is Head Girl of Y11 and would have a chance to continue as HG in sixth form if she stayed on.
I feel moving schools they may not be as 'recognised' and would just be average. The teachers wouldn't know them and genuinely care for them in the way they do now. However, maybe the current school can't stretch them as much as another school could with 'higher calibre' students. (Sorry if that sounds snobby!).
Interested if anyone been in this position and has the benefit of hindsight. Or, teachers who work in schools like DC's current school and what you think.