I went to a catholic school and while I'm not against the idea of the kind of compulsory RE many posters are describing, mine was not that at all.
Mine was almost completely about Catholicism with a bit of CofE and some Judaism and Islam thrown in, but only in the context of "here is where they agree with us" about various issues and "here is where they don't".
I didn't learn anything about their festivals, rituals or even their beliefs independently. I had no idea about Sikhs or Hindus. It definitely didn't help me respect anyone else and their religion.
I have memories of my RE teacher going off on tangents about how we shouldn't be promiscuous because no men would want to marry us and making the boys in the class tell us the same. "Everyone loves a slut but nobody wants to marry one."
I have memories of lessons about abortion where we looked at graphic images and had abortion methods described to us in detail. Only surgical abortion of course. We were told all about how much pain the foetus felt. We were never spoken to about terminations for medical reasons or early term abortions. Or women's bodily autonomy. Or the dangers of childbirth.
Sometimes we were told different points of view but it felt like a box tick exercise and was done through a very specific lens.
"Such a such a group thinks abortion is ok in some circumstances however catholics believe that murdering adorable babies is wrong. Now let's watch a video about baby murdering. Obviously some of you might think it's ok to murder babies and you're entitled to your beliefs but all I can do is give you these facts. Ssh if you listen closely enough you can hear the babies' hopes and dreams being destroyed."
When asked about abortion after rape, we were dismissed instantly with "it isn't the baby's fault."
No acknowledgment of the difficulty in making such a decision or the complicated situations that arise or even where you can go for support related to the topics.
I got an A* form figuring out that catholics thought everything is bad. Abortion bad. Sex before marriage bad. Contraception bad. Euthanasia bad.
Yes it's a catholic school but telling a load of 15 year olds that a woman's worth as a person is tied to their virginity, that enjoying sex safely will make them damaged goods, that if they get raped and have an abortion they are murderers etc is simply abusive in my opinion.
It's for that reason that I am wary of compulsory RE. If it's genuinely learning about many different religions in a respectful way and being given various points of view to consider, then that sounds great. I want my child to understand and respect others.
But I would be horrified if they were RE lessons like I was having.
I cannot imagine sending my child to a catholic school in the first place but if it was the only decent school around and I did so, I would be very interested in what was being taught and very quick to tell them to take no notice if necessary. Even if it meant them failing a GCSE.
So while I have sympathy overall for your plight, I am quite interested in the curriculum you're actually teaching and whether it is parents being disinterested or if it's parents being against what's being taught.