I remember the explicitly non-Christian pupils in my junior school always skipped the hymns during assembly and never had to go to talks about Christianity but were never called racist and nobody ever really cared (except for maybe jealousy that they didn't have to sit through that cack).
That was the annoying thing - most schools fail to recognise non-belief... if your parents are Christian then as a kid you're forced to attend religious assemblies if you go to one of the nominally C of E or Catholic affiliated schools. Other kids with parents who happen to be Muslim, Hindu, Sikh or Jewish are automatically given a free pass to avoid this nonsense. Kids aren't stupid and a large portion of kids being forced to sing hymns etc.. are fully aware that its a load of nonsense... yet schools carry on wasting valuable time with this sort of thing.