Might be the wrong thread, seeker but since we're talking about equality in education... 
I went to a catchment mixed secondary at 11. There were two single-sex schools (both ex-grammars and run by the old grammar heads along similar lines) in the town that ran 13-18 and so there was a voluntary transfer after the second year (Y8). So I've been to both mixed and girls'.
But, at 13 after two years of streamed/set classes at the 11-16, it was obvious to pretty much all pupils who were the academics and could cope with the usual tranche of academic subjects, and who would prefer to stay at the schools with the large wood/metalwork/tech drawing/home ec/needlework facilities but still do the main subjects too and have option of CSEs or O levels (dates self!).
So it became a matter of parental/child choice to go to the more academic comp but it was in essence self-selection. And it worked. Those who were late developers at the 11-16 could transfer into the 6th form at the 13-18s which was independent of O level or CSE attainment or go to the 6th form college.
And then the council decided to scrap it because the two 13-18s were still being run like single sex grammars and that was elitist and would never do, would it? sigh The boys school is now a sports specialist academy, where before it sent half a dozen to Oxbridge a year. Girls became the education dept HQ.
So I'd say that mixed high schools from 13-18 might well work. Make them primarily academic (don't have most of the vocational stuff, keep art and music and food tech and PE) and let people choose to stay at a more vocational school or move. Kids know. They'll pick the one they're suited for most of the time, and they can move later, freely, if they want. But with no invidious 11+.