Have now been to four school Sixth Forms and a clearer picture is emerging in my mind of what's going on. Grey Court seems to be doing well and retaining a high % of its own students post GCSE. It's not yet full but has 100 in current Yr 12 I think. It seems to be offering a slightly broader range of courses than the other schools including BTECs.
Orleans Park is also doing fairly well at retaining its own students.
Teddington has fewer students than the others and I know it is suffering from the long tradition of everyone going to Esher. It has the smallest building of the four so possibly couldn't take as many students anyway. I noticed that there were quite a few Hampton High students at the open evening.
All three of these schools are saying that most students will from 2017 be taking 3 A-levels and not doing AS.
Waldegrave was full last year, turned 24 people away, and is capping its intake at 140 in 2017. After Waldegrave girls, of whom apparently about 80 stayed on last year, there are 40 reserved places for boys and remaining 20 or so places on distance. All the boys I spoke to had come there from private schools, although I'm sure some of the boys there are from state schools. The head boy (from a private school) gave a speech about how nice it was to be in a place where he was not just a grades statistic and he felt people cared about him which my daughter found quite amusing as her initial impression of Waldegrave was that it was obsessed with results! I feel that it is covertly selecting because, unlike the other three, everyone has to do 4 subjects in Yr 12, take 4 ASs and then drop one subject and do 3 A-levels. This is off-putting for the slightly less academic students (and for lazy ones!). We were also told that "anyone who wants to do BTECs should go to the Hampton High table" (Waldegrave, Hampton High and Teddington were all represented at one another's open evenings)!
So, although the entry criteria for all four schools are the same ie. you generally need a B for you chosen subjects and 5 A*-Cs, something a bit different is happening to the intake in practice! When you see next few year's results do bear this in mind!