Nowadays the majority of people do not have any religious beliefs, nor do most chilren. They are completely ignorant of what religion is, have never been in a church, temple, mosue or synagogue. To expect them therefore to latch onto the idea, the hymns, prayers and readings they would get in a school assembly, is unrealistic - and what about the schools where te majority of children may be being brought up in a rligion, but not christianity?
I think schools do need to have assembies to build up a sense of unity, but if it is to be more than just a reading out of notices, then whatever is done should be non-religious. I am sure there are many themes an assembly could be built round, with poetry, readings and music that are not religious.