Sure it’s an option, but it’s not the sort of option that the government would need to mandate in my opinion.
The religious based curriculum is an attractor for a lot of these schools and desired by a lot of the parents. I bet in most cases it is a deciding factor of the parents. Because of that I would suspect most schools would be unwilling to carve it out and make it optional. They have enough student market available to them without having to change the product they provide.