A school can make ANY subject it likes a 'core' GCSE (ie one that all students take). It's up to THEM not the government (although they can't opt out of the ones the government DOES rule on, such as English or Maths). You'll have to find another school if you object that much.
You do realise that RE (or Religious ethics and Philosophy as it's more widely known) is a FANTASTIC GCSE to prepare students for A Level/uni don't you? I'm a history teacher and I hugely value the contribution that RE makes to students' ability to argue and write coherent, meaningful essays.
They also debate ethical issues that society NEEDS them to consider such as abortion or euthanasia... why on earth would you not want that??
The religious aspect is important too. We're looking at terrorism in the modern world in history at the moment, I can't imagine trying to teach that (surely one of the most relevant subjects I can teach in history, in terms of students' lives in the modern world) with a group of kids who DON'T know the basic beliefs of and differences between muslims, christians and jews...
... not to mention the other religions that they study.
Again - why WOULD you not want that? Studying world religions is VERY different to 'doing Christianity', which is NOT was RP&E is these days.