Filee is coming over as one of those parents who think that teachers should be dealing with each child's problem on an individual basis despite the fact that his stupid mother fucked up in the first place and expected the school to be mind readers when she changed her instructions to the school.
I don't know what happens in church schools, but I have experience of county/community state schools both now and 30 years ago and Christmas was about as religious as it got in primary school. (not saying christmas wasn't religious, just that there wasnt more religion at other times of year). If you can understand Christmas in a non specific cultural way, I don't understand why the rest of it couldn't be understood in a non specific, 'lets all be nice to each other', 'loving your spin on the old pagan rituals way' too.
I mean this is the c of e we are talking about.
(Unless this is a catholic school, in which case OP should have paid more attention to prospectus)