I can think of several young people who socialize more with other faiths and cultures outside of school than they ever do in school, so I'm not sure that your argument is watertight, ippy.
Sure there will be kids you socialise outside of schools in a broader manner than in school. But that isn't an argument for what is effectively 'social engineering' creating structures that act to divide kids into various religions and the rest.
But it is pretty difficult to argue against Ippy's basic point.
So to use an example - my kids go to a state non faith school - the intake largely matches the demographics of the local area, so the people they learn with are reflective of that community in all sorts of manners, including religion. So they know and socialise with loads of kids who are from non religious backgrounds, plus others who are active CofE, RCC (plus other christian denominations) and also muslim, sikh etc etc.
My sister in laws kids attend a catholic school with the intake overwhelmingly from the catholic 'community' of a large city. So even though perhaps only 10% of that city are catholic, perhaps 90% of the kids they socialise with at school are. Their school is largely devoid of other faiths, and certainly their impression (from what they see at school - and actually at home too) is that the world is largely catholic, with a scattering of non catholics.
It is pretty clear which schooling provides a broader opportunity to socialise, interact and learn about kids from all faiths and none - plus also to understand the proportions of those people within the broader community.