@WombatChocolate at LEH DDs year group in GCSE year told not to fret about being kicked out before sixth form . so it doesn’t happen everywhere . Head said all the girls were capable of doing well ( presumably because very selective !) but if a girl didn’t do well in GCSEs then there would be a good reason for it, so there was no internal bar to pass on into the sixth form . Head said it was primarily to “stop the chat” that those who didn’t score high enough would not be able to stay on and reduce the pressure on the girls . Contrast with DS selective school where some boys got warnings that their sixth form places might be at risk. In any case Most schools ( state or indie ) have a minimum bar to go into sixth form so it’s not unusual for culling to happen . I believe Tiffin Boys removes a chunk of the lowest ranked GCSE boys to make way for the 6th form girls ( who typically have very high prediction grades ) do culling isn’t a preserve of the indies .
A lot of girls move from girls to coed for sixth form such as Westminster and KCS. They are typically some of the higher achievers which they would need to be to get in to the likes of Westminster . This is why sixth girls at some of the top girls schools tend to be a bit smaller .
there us certainly a lot to digest when you choose a school - and sometimes when it gets tight the silliest things make an impression . A close friends DD threw out a scholarship from a lovely school on the grounds that the toilets had been blocked on interview day !