I have extensive experience of the German 3-tier system and frankly, the UN can get stuffed on that issue.
German grammars work well because they are virtually disruption-free and teachers are required to have not only their teaching degree, but a good degree in their chosen subject(s), too.
German middle tier schools work well, because students leave education at 16 and are not forced into an academic route after that.
German Hauptschulen (the bottom set tiers) work well, because rather than having to focus on academic abilities, businesses get involved and students learn actual, practical skills and gain qualifications in manual labour, still leading to a chance at high-earning jobs, such as plumbing.
The comprehensive equivalent are often shit schools, because throwing everyone together doesn't work and teachers are simply suited to work better with one set of abilities than another.
Who honestly gives a crap that "social mobility" targets still doesn't force a more equal picture; at some point we just have to accept that there is a genetic variant to being bright and having academic skills, and try as you might, some students will never achieve a grade 9 (my head should take note; they are of the opinion everyone can get 9s if only they try hard enough and if teachers were better).
The system is flexible, too, so students can swap between school types if they turn out to be in the wrong one.
Whereas in England, we have
-top set kids whose learning is highly affected by behavioural issues, often by kids who are lower ability, but are stuck into these sets to spread behaviour issues out a bit, but cannot for the life of them access the speed and quality of learning
-bottom set kids who are set up for failure because we have to force them through academic qualifications they neither want nor need and who just cannot access the work, even with less content, because there is not enough time per subject to make a dent
-middle set kids who are pressured to destruction because they have to get those fives for the school to look better in league tables and who often vote with their feet as a result
And lastly
-schools who are grammar schools in all but name because forcing them to become comps just meant parents were moving areas to schools with a better reputation, leading to two schools literally across the street from each other becoming a grammar equivalent and a sink school, or some areas not having any decent schools at all
So I'm all for bringing grammars back. I'm also for completely ditching what we do in the bottom tier. We should, instead, work with businesses to encourage those students into apprenticeships - by all means carry on with English and Maths, but ditch what they don't need and focus on functional English and Maths skills. BTECs have become far too academic to be of any use and I can't see any school age actual technical qualifications which help those kids.
And teachers should be able to choose. I'm great with both top and bottom set kids, but I'm fairly crap with middle sets, because they often contain the worst of both end of the spectrum and my ND brain can't cope with that.