The same could be said of a state grammar, really, with perhaps the proviso that there's possibly going to be less 'intensive intervention' for the DC who ISN'T coping as that costs money (for the school)- the privately ed'ed DC's parents might be asked or told to get extra private tuition under those circumstances? I don't know!
No, I am SO not starting a state v. private debate here , but I am wondering whether this (the OP) is what privately paying parents feel they're getting?
I know someone well who has well, I suppose 'blamed' her relative 'lack of success' (I don't see it that way as she does the same job as me!!) on her schooling; she now, courtesy of a DH in banking, has her 2 x DSs in academically selective private schools and part of her rationale IS that idea that at least she'll KNOW they're being MADE to do their best.
I also speak as someone who MAY put a DS into a small class private for 2 final primary years to attempt to get him up to speed so I'm not criticising!