https://comprehensivefuture.org.uk/comp ... n-mislead/
This is from a group which is not impartial, but does use and quote much impartial research. The key reference I think for what you are interested in here is from Jesson (2006).

This goes beyond the inequity arguments which people like me usually cite against selective education. There is more recent research demonstrating the same thing but I can't instantly recall who did it or quote it here, sorry.

Has any research Organization done the research about bright children leaving primaries and their achievements on leaving the 6th form in non-selective school vs selective school?

Ofstead Chiefs have on more than one occasion recently have called comprehensive schools as having failed bright children. Why would they make such suggestion?

Other than Bucks and Kent, which areas are organised on selective lines? Is just having one or two grammars in wide catchment area makes it selective area, eg Kingston? Local intake in Tiffin is too low due to wide catchment and remaining Kingston schools are as comprehensive as to be found anywhere.

And what is the performance statistics of rest of the 137 totally non-selective areas? Are bright children there achieving their full potential?

This is controversial and I can't say where but my lodger is an English teacher at a comp. It's a low performing comp (non grammar area) situated in a very mixed, mostly lower social class and white ethnicity town. I'm in the South. They are in the process of removing setting streaming except for Maths. The main reason is to improve "LOW" level disruptive behaviour in the class. She admits she won't be able to appropriately stretch or give enough attention to either side of the ability spectrum but it will mean her class will become more manageable as the disruptive kids no longer are in the majority. Coupled with the fact they have been without permanent teachers for key subjects they simply, as a state comp are unable to cater well to everyone. She as a teacher will not send her own son to this school and is electing for grammar or private. Not because she believes it is the best way to educate but because she views the current default as failing. She plainly admits she would remove the bottom 10% who have no interest at being at school and are extremely disruptive elsewhere just to give the 90% rest a decent chance to excel but they can't do that as they are forced to taken an inclusive stance and importantly, they will lose funding.

My opinion is that comp education works well where there is wealth where there is vested interest in going to a school for the standard curriculum. For poorer areas, a range of specialist schools may be more appropriate.

10% is a max of three in any class - a competent teacher should be able to manage that. A lot of the poor behaviour is down to an inappropriate curriculum. I've had to teach Year 10 bottom set for a double lesson on a Friday afternoon so I have experience in supporting lower ability groups. I just devised activities so they learned maths without it being obvious; they all got a GCSE at the end of Year 11.

Primary..but I taught a class of year 3/4 non readers. Had an observed lesson where we played board games I had made. Inspector chuckled about the fact that he overheard
“ Sshh, don’t say anything, but she hasn’t realised this is supposed to be maths lesson”
mwahaha!