How compulsory is RE post 15 years old?

Talkinpeace - you're quite right that parents can withdraw their child from RE but I'd be surprised if your daughter's school undertakes that administrative burden for every child.

RE is compulsory to KS4 (www.gov.uk/national-curriculum/key-stage-3-4) but there's no requirement to sit an exam or to deliver the curriculum through discrete RE lessons. I venture that your daughter's school is perhaps delivering the content via PSHE or similar.

DD is year 10 at a state comp.
She DOES NOT have an RE lesson - nor do more than 70 kids in her year.
When we did the GCSE options, RE came in the same timetable category as PE
but dropping it left more space for Latin
so we did.

I repeat
RE is NOT compulsory at KS4 so long as the parents sign the form

it is not compulsory at all at Sixth form college (KS5)

RE is compulsory up to 18, I some form. Most schools have it up until GCSE and thn deliver it in other ways if at all in sixth form.

PHSE is poorly taught in most schools and doesn't lead to exams.
Both of my DSs have done RE, both atheists and both have loved the GCSE curriculum because it encompasses exactly the things you mention that your DD is interested in - morals and ethics.
Having said that , at their school any child who is struggling towards GCSE exams is likely to drop RE in order to concentrate on the core subjects.

DD chose RS as one of her GCSE options (she's also doing History, but dropped Geography like a hot potato!). She's not in the least religious (neither are we) but it's one of her favourite GCSE subjects as she thinks it is more open-ended than many of her other options and really allows her to think. She also recognises that religion, though irrelevant to us personally, has a great bearing on so much that is happening in the world.

I wouldn't be so quick to dismiss it-and I say that as a rabid atheist.

Science is also compulsory, a language is not but is recommended.

Both of my 2 boys did it to GCSE level. They did they short course over one year at their school, and then do Critical Thinking in Yr 11. It goes deeper than 'just' religion. There is a lot of philosophy and ethics and they learn how to see things from different points of view.

We aren't religious in any way at all, but we saw it as beneficial to our DC.

My ds1 has to take a GCSE that co ERS citizenship and RE, he is in yr 9 and its compulsory in all the high schools here.

Although RE is not part of the national curriculum, it is a legal requirement that every pupil in key stages 1-4 has one hour of RE a week unless they have been removed by parents for religious reasons. School will have a policy for that happening. In my school we say that parents need to come to school and supervise their students whilst they ate meant to be in
RE if they wish to withdraw them.

RE is compulsory at KS4 (so up to the end of yr 11), so most schools will expect students to take lessons called something like 'non-exam RS'. Some schools, therefore, make the students take a full or half GCSE in RS since they have to be studying it anyway. At our school , they all take non-exam RS and some also do a full GCSE in it, too.

I thought it was a compulsory part of the NC to end year 11, along with PE, and PSHCE, although obviously not compulsory to take an exam in it.

DD dropped it for years 10 and 11
DS will too

I don't know the answer to this but DD is in year 9. She hated RE at Middle school but has loved it this year at Upper school they have been doing Philosphy and Ethics which she's really enjoyed.