Where I grew up there were non-denominational secondaries that generally weren't very good, and faith schools that tended to be better (also a private school but the chance of me going there, even via assisted places, was roughly the same as sprouting wings and flying the family to Barbados). My parents moved half-way through the school year, so my elder siblings went to a non-denominational school and it only took six months or so of my elder brother truanting and the school not actually noticing or finding out until my mum's friend spotted him in town for my parents to decide they were sending the baby to one of the Catholic ones. My family are Catholic (though I ended up getting baptized C of E for daft reasons) and they liked a clever kid, so there was no having to get a priest sign anything unlike the C of E school in town.
Obviously this was a long time ago and may be very dependent on where you live, but having to take compulsory RE was not great - partly because there was an expectation that you knew a fair bit about the tradition, partly because our area was 30% Muslim with a lot of racial and religious tension, and yet we only studied Catholic tradition and teachings when it would have been far, far more useful to have this in the context of other religions and how those intersect with "ours". Sex education also wasn't great - again, this may have changed, especially as I was at school under Section 28 and there's probably more focus on consent and emotional aspects now, but it was basically 'here's how your body changes, here's how pregnancy works' in Year 7 and then 'this is why abortion is wrong, this is why contraception isn't allowed' in Year 11. Sure that's something you can remedy at home but not all parents will or know they need to, especially in an area where teen pregnancy was fairly common. I honestly learned more from Just Seventeen about it than school.