What you have to remember is that there are a LOT of selective and non-selective prep schools in this area.
At the fireworks on Friday night on Trinity Fields there were over 4000 people in attendance and they were organised by the "consortium" which is a group of private schools (Thomas's, Broomwood, Northcote Lodge, Finton, Hornsby) plus you have Eaton House and Parkgate and Olivier House and the White House and Newton Prep and Wandsworth Prep etc etc etc .
So there is a HUGE body of primary school age children in this area most of whom will either go away to boarding school or to a London day school.
If you take out those that board (I have no idea about the % and it will vary from school to school but I reckon 30% on average. Higher at traditional schools that have 13 as a leaving age, much lower elsewhere) then you have a massive "pipe" of kids hitting 11 years old who need to go somewhere.
On the whole they end up at JAGS, Dulwich, Alleyns, Emanuel, Whitgift, S&C, The Hall, Trinity, KCS etc etc etc and ALL of those schools have entrance exams and interviews.
The independent school kids are prepared and coached and trained so that they will get into one of these (or similar schools) and the prep schools will be also training the parents and managing their expectations. They'll be meetings with headteachers where it's suggested that "little johnny" doesn't consider KCS but does think about Emauel as his grades aren't up to it and the heads all have lunch together and brief each other on their pupils and who would "fit" best where.
It is a machine.
A great big machine where the cogs and wheels whirr and click to make sure that each child ends up in the "right" school for them.
That is what your paying for.
On results day, if there is a "shock" (ie a child doesn't get an offer from their preferred school) the phone lines between prep and secondary will burn up with calls as the prep school heads "sell" in their children and by and large, it all works.
Some are disappointed but most are happy and almost every kid ends up in the right school.
And most of these preps are non-selective ie anyone with money can get a place.
Now contrast the state child trying to get in at 11.
They have no support from their school (nor should they).
The bright ones (and I mean REALLY bright) will be snapped up by the independent secondary schools. They'll help move their grade point average in the right direction at GCSE and they'll nail the exams.
Of course they will, they're bright.
But the kids who are "normal" (and I mean normal - not below average) will struggle to get a place. They'll struggle because they don't have the machine helping them on their way and they probably won't get in.
Now because there are SO many of these state school children applying there are enough REALLY bright ones to get in so schools like Emanuel can claim 70% of our intake are state and make it sound like anyone at 11, no matter they're background, will get a place, but it's not the case.
So, phew, in summary
1) less able children who are privately educated will stand a good chance of getting in to a selective independent secondary because thats what they're trained for
2) bright state kids (really bright, please don't confuse just being top of the class, that's not enough) will be ok. They'll nail all the entrance exams
3) normal and less able state kids will struggle.
So Thomas will send lots of it's kids there because Emanuel will take a great deal of their intake anyway.
The bright state kids will go there because it's a great school.
Its the poor kids whose parents can't afford the fees of a prep but who are just normal who won't get in.
Does that help?
Does that help?