I totally, completely get you OP. In fact, from your OP I initially suspected you were talking about DS's school - but we're even further south and west than you and another 20 years behind.
Let me guess...taken on its own this might be OK, but there's the whole drip drip of monocultural assumption behind it, perhaps? No, our school doesn't celebrate Diwali. It doesn't celebrate Eid. I'm pretty sure the kids have zero idea that these festivals exist. Meanwhile, there's a lot of sneaky promotion of Christianity, to the point that you could almost call it evangelism. Without any of the parents' prior knowledge or permission, the kids have been being bussed off to church to attend some kind of Christian service/ activity- now my 6 year old DS tells me he's a Christian and I shouldn't be saying "hell" because it's a rude word!
There's a "British Values" display in the school, with photos of the Royal Family and Winston Churchill. That's it. Not a single Briton who isn't white and upper class.
And what are these "British Values" anyway? Tolerance and democracy? Because all furriners are intolerant and undemocratic? Right....
Perhaps a good costume would be a 19th century colonialist in shorts and pith helmet?
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Wish me luck for Monday- I'll be having Words at the PTA meeting about this whole Christianity thing!