My DD goes to a Catholic school, so we expect prayers, and we knew about it when we chose, which is fair enough.
Normal schools should not have to have collective worship. OP is right. It is not on to not have the choice to send your child to a school where there is no religion.
When I was at a mainstream school as a child, we had someone come in once a week and tell us that evolution was impossible, that sex before marriage was a sin, that nonbelievers would go to hell and that prayer could heal. Which are all fine IF you beleive them, tolerable in a church school, but not the kind of thing that parents expect when they have chosen not to send their child to a church school.
I don't like the fact that DD goes to a Catholic school (although I love the school itself) but I just have to deal with it and let DD join in, because what would be the point in going out of our way to go to another school? At least at the catholic school we know the set of beliefs we are dealing with. We support the catholic stuff, we just don't join in, but that is because our family is pretty mild atheist. Families should be able to trust the school to teach only verifiable facts and skills that are relevant to all, and prayers etc should only be in schools clearly labelled as religious.
I'm a socialist feminist - we sing The Red Flag at home, we learn about related history and values, the kids dress up as suffragettes, we take the children to marches and so on. I would consider sending them to a overtly socialist feminist school (...you get what I mean, don't get pedantic) but I wouldn't expect mainstream schools to start foisting our family beliefs on other people. Arguably, politics has much more relevance to daily life than religion, so why does religion have such an unavoidable place in education when it is becoming more and more of a minority interest?
I'm not advocating missing out the cultural and historical stuff - I take the kids to see cathedrals, we learn bible stories and to be respectful of other belief systems, but we also learn about Islam, ancient Rome, local legends and so on.
I have no objection whatsoever to people practising their own religions. They can do what the hell they like, and so can their children, as long as I am not forced to join in. I might choose to join in (eg by sending my child to a Catholic school) and by making that choice I have also chosen to do what is needed (eg allowing my child to pray etc) but it should be A CHOICE.
Religion (or lack of) should NEVER be forced on someone. End of.