The problem with religion AS FACT in schools is, that it is a topic of faith. We can by definition not know what and if anything is true about them. There is no evidence at all.
You are free to believe what you want, and education about what religions are out there and what their core beliefs are is fine, too.
But the moment you teach something as fact you better have natural evidence for it.
And a scientific education is the only one where that can be made sure.
The moment you present evidenceless beliefs as equal to science you lose the seperation of state and church so a secular school is the only one that has validity.
Not to mention, which faith do you plan to teach as fact? Christianity? Islam? Buddhism? Hinduism? Sikh? Cargo Cults?
Either all of them or none....
If you teach all of them you render every single one invalid, because they contradict each other.
That's why we only teach things as fact in schools which we can in fact know, because they are observable and verifiable.