Withdrawal from RE

That is the accurate information. The whole "they'll ask you to leave" thing was just something a teacher said, and that's it. Everything else is accurate, and now that you know the circumstances around that specific point it's been cleared up as well, although I apologise for not clarifying earlier; I only found out the vast majority of this today myself.

I wouldnt take notice of what an individyal teacher said and would seek verification from the school. That being the headmaster or at least the head of sixth plus where they obtained the authorty from.

Why wouldnt they be able to make you leave? You join based ion terms you agree to. they should be on the website. If you dont have them, then it would seem sensible to ask the school for a copy.

Its their school. take it or leave it.

Though your issue is about having to study an actual subject, the fact that you would've had a free otherwise is kind of like what we are being made to do. At our school, we have something called CAS that everybody has to do unless their timetable doesn't fit in with it. It is essentially choosing to do 1 out of 3 extra curricular activities for those who do not have a lesson p6. Those that do have to do an alternative sports activity but some ppl (including me) don't have frees on the periods that those alternative activities were scheduled so we're are exempt from it(!) Most ppl hate it as they would've have been able to go home (or wherever else ) if this didn't exist and this wasn't actually specified in a contract though internal students like me may have heard of it
Unfortunately, the school is legally able to force such things on us as it would help us when writing personal statements and CVs so ur school may have some similar intention
Talk to ur headteacher and maybe start a petition? - although, this may be a little extreme !

Though your issue is about having to study an actual subject, the fact that you would've had a free otherwise is kind of like what we are being made to do. At our school, we have something called CAS that everybody has to do unless their timetable doesn't fit in with it. It is essentially choosing to do 1 out of 3 extra curricular activities for those who do not have a lesson p6. Those that do have to do an alternative sports activity but some ppl (including me) don't have frees on the periods that those alternative activities were scheduled so we're are exempt from it(!) Most ppl hate it as they would've have been able to go home (or wherever else ) if this didn't exist and this wasn't actually specified in a contract though internal students like me may have heard of it
Unfortunately, the school is legally able to force such things on us as it would help us when writing personal statements and CVs so ur school may have some similar intention
Talk to ur headteacher and maybe start a petition? - although, this may be a little extreme !

You are tagging the wrong person.

"it's a religious school"

Much as I'm against enforced religion, if you have chosen to go to a religious school then you're probably stuck with it. Our local (C of E) one makes parents sign something about agreeing with the wonderfulness of Jesus (I paraphrase) when students join. From my experience of Roman Catholic ones they're even more keen on pushing religion at all possible moments.