Most state secondary schools (at least in my area) no longer have a sixth form. Students who wish to do A levels, or other post GCSE studies, go to a "Sixth Form College" or "College of Further Education". These studies are free up to age 19.
It depends where you live. Here in Reading there are at least two state schools that still have sixth forms. My children both went to them. I think bhaisahab's 'most' is probably right at a national level though.
I think that, as far as this translation is concerned, you could say 'go on to study at sixth-form level', which covers both cases.
b
PS Colleges of Further Education also offer GCSE course (usually retakes), and 'post GCSE studies' include both studies of things at a higher level than GCSE and practise of other things after failure at GCSE. I know, having taught in one (and I could show you the scars to prove it!)