Thanks for the replies, it sounds extremely complicated.
There isn't much reference to the School Cert. in the CS books, except for a bit at the beginning of Highland Twins
where Robin gets a letter from Elizabeth Arnett detailing her results:
Robin glanced hastily through the sheet of scribbled writing. 'Yes; credit in English, French, and botany; pass in maths and Latin. Oh, well done, Elizabeth! And she's heard from Biddy, Nicole and Myfanwy, and they've all passed too. Biddy and Nicole have distinction in French and Myfanwy has credit in Latin and botany. She doesn't know about the rest.
And in Rescue,
Jo says that Reg needs "School Certif. with matric" in order to go to Edinburgh to study medicine. So taking him as an example, and if I've understood everyone correctly, he would have taken the School Cert. at 16, so probably Upper Fifth or whatever Polgarth's equivalent was, and would have had to have taken six subjects including English, maths and at least one science or a language, and would have had to have got credits in at least five of them in order to matriculate? And then he presumably stayed on at school for another couple of years, possibly doing the Higher in his final year, before moving on to Edinburgh at 18? (He would have been in one of the last year groups to do the School Cert. before it was changed to GCE.) I'm guessing he would have taken subjects like chemistry and botany and probably Latin, since he was planning to study medicine.