Dunno about primary schools, but middle schools are supposed to teach about different faiths as part of RE. Ideally part of the curriculum should involve visiting mosques, churches, Gurdwara, synagouges etc. but of course this is restricted by local availability and time in the school day. Even adult atheists can learn a lot from talking to practitioners of particular religions.
English schools do not necessarily have a Christian bias. The enthusiasm in observing a particular festival can depend on the predominant religion of the pupils at that school.
Most people quite like a bit of pageantry, and that’s what a lot of religion is. You can have a bit of fun without actually believing.
Its the Jehova’s Witnesses kids I feel sorry for. They’re not allowed to take part in anything.