Surely a solution would be the big school (ie having the cash to provide a wide range of opportunities) with 'x' % of pupils of 'A' grade material, 'x' grade B, 'y' (or whatever) of Cs etc.
Chuck 'em in together then stream, stream, stream! Provide 1st class science laboratories, sports stadia, performing arts complexes, metal and woodwork-shops. And even a working farm! AND (importantly) a 'pupil referral unit' where all disruptive kids would have to go and could easily be sent. Prize giving would award a prize for 'best-in-school' AND 'most improved' across ALL subjects.
This can be acheived. My husband's rural state high school in Australia got kids into university to do chemistry and kids whose animal husbandry skills won County Show prizes.
FWIW all the secondary moderns in Salisbury make mention in their OFSTEDs how the school can only do so much because the most able kids are creamed off into the state grammars. Many of the grammar kids were privately prep-schooled to ensure their place- there's a reason why Salisbury has no boys private secondary school and the girls' one is RC or another that is frighteningly expensive, yet the town is bursting with prep schools!
Like UQD I was at a (girls'!) grammar (in 73). It was an excellent, academic school. The secondary moderns were, by and large rubbish, teaching watered down academia. This is why the SM/GS 11+ thing failed: No one would grasp the bullet and offer the less academic kids real alternatives, and there was no swapping between schools, once you were in the grammar, you were in for life, ditto the SMs (there were girls in my class who sat CSE maths and got level 3s. They, IMO, shouldn't have been at the grammar at all.).
I want my boys to go to a grammar. They won't because they're not clever enough to compete against prepped kids, but actually it isn't the academia I specifiaclly want at all, it's the focused learning ethos, the high value placed on education, the knowledge that if you mess around too much, you're out. It's the celebration of effort and success. You have to pay a high house price to buy that in a comprehensive!