I taught for many years in primary schools, and because I had caring responsibilities for a few of them, spent several as a supply teacher so I saw a lot of schools.
I can totally see the OP’s point and I think those who don’t are being a little obtuse.
Yes the curriculum requires pupils to learn about different faiths, but there’s usually a distinction between how the others are taught to how Christianity is taught. It can be subtle, though often it’s not, but other religions are generally taught as ‘Muslims believe’, ‘Hindus celebrate in this way’ whereas Christianity is often presented as if it’s just true, or the norm.
As I say, it can be subtle but not always.
I have no issue with my daughter being in a church or knowing the meaning of Christmas from a Christian perspective. In fact we attend at least one church service over Christmas with her - this is for the sake of a family member who stays with us but also because DD enjoys it and I think it’s a nice tradition. Some people will think that terrible as we’re not true believers/ regular church goers but we’re certainly not on our own there and I’m ok with it. I still think her school’s job is to present Christianity as one faith among many, and not necessarily the norm/ the only truth.
I accompanied her class on their trip to the church recently and the Vicar was very nice, but absolutely spoke to the children as if what he was saying was categorically true. He’s also a governor at school and goes in to do assemblies at which the children say prayers and sing religious songs (all a bit newfangled so I’m not sure they qualify as hymns!) This is not a church school. Other religious leaders and parents of other faiths do also go into school which is great, but they are the ones that talk about what ‘we believe’ or they’ll tell the story of eg Diwali very much as a story.
During this church visit the children were taught one of the ‘child friendly’ Christian songs that apparently the vicar is going to be using in his assemblies going forward. The main message of the song, explicitly stated repeatedly in the lyrics, is ‘so I won’t trust in myself I’ll just believe in God instead’. I think that’s an appalling message and a massive overstep to be teaching children that (the majority of whom are not Christian). It’s been on my mind to write to school about it, but I too have a horror of being ‘that parent’.
I can absolutely see where the OP is coming from.
Also, every school I’ve been in, if they had the blanket consent thing, would still inform parents about any upcoming visit that meant leaving school grounds. I’m surprised that there’s some that don’t. Usually they need helpers, as the church above. But otherwise it’s just good communication - which schools are also required to demonstrate.