Even in later books, specialization seems to be mostly restricted to girls over 15 - until then, they're expected to keep a fairly standard course of study.
Eight subjects to O Level, thus to age 15/16, was the norm when I was at school, and I think it was for School Cert previously too.
We were required to do maths, English Language and Literature (two separate subjects), French and one science - biology, physics or chemistry. We could then opt for three other subjects from those offered, and within timetabling constraints - I'd have quite liked to do German, but it wouldn't fit in with the other subjects wanted to do and was required to do.
For A Level, at 17/18, we dropped down to three subjects; that was when I dropped all maths and science.
The CS specialisation does seem to be a bit odd at times - I don't know why Jo was still doing maths and art in the Sixth, when she had no aptitude or interest in either of them.
Then in Challenge
, Evelyn is said to be sixteen and to have done her O Levels, so she should be in the Sixth and specialising, but she's put in the Fifth with people who will be doing O levels in the coming summer, so will presumably spend the year repeating work she's already done. I can imagine how I'd have felt if I'd been told I had to do another year of maths when I thought I'd finished with it forever!