I don't know if it's so much about explanation Elegran as just meeting other cultures in a 'normal' way. Where I live it's very multi-religious; Muslim, various Christian sects, Sikh, Hindu, Judaism, etc. and multicultural; Asian, Eastern European, black, white, etc.
When I've had friends visit I've been shocked surprised that they notice the diversity as we are so used to it. It's also been an eye opener to thrm that the Musilm chappy who owns the off-incense, speaks perfect English with a local accent, has a great sense of humour and is just like us. This is what the children in non faith schools are learning on a daily basis.
And as far as practising their religion I don't know that going to a non-faith school has any effect either way. I'm sure the Sikh boy in my GS's class goes to the temple still for example.