Johann Hari, who is generally lovely, sidestepped the primary school review thingy and has this to say about secondary schools.
It amounts to a forced mixing up of backgrounds in state schools (so you don’t get post-code apartheid). A quota of free-school-meal kids has to be filled but cannot exceed 40%. Mixing it with the Guardian classes is apparently enough to raise all boats.
The Netherlands has a system where schools get more funding for taking on free-school-meal kids – the idea being it would force all schools to take a few but not so many that it drives results down.