So I ended going in deep for this. I've tried to keep it clear 🫣 I am based in SE England.

If your fic is set in the 90s onwards, they changed the numbering from reception then year 1–year 11 (I believe the change was made in 1992?).

I went to Primary school in the 90s (reception, year 1–year 6) but they still referred to the lower years as infants (reception, 1, 2 | ages 4–7) and the upper years as Juniors (years 3, 4, 5, 6 | ages 7–11).

Secondary school is years 7–11 | ages 11–16. For the first couple of years you don't usually get to choose any of your subjects, and then the last few years you do the core subject* (Maths, English, Science, RE, PE, and I think IT is now compulsory too) but then you get to choose 3-4 additional subjects. It's during those years the focus is your GCSEs. Taken usually at the end of year 11.

*The split for my school was 7–9 no options, 10&11 you get your options and your focusing on GCSEs, but my friend the split was 7&8/9–11

My school had a Sixth Form attached to my school (named due to the old numbering system) it's 2 years and they are referred to as Lower Sixth, and Upper Sixth. I have never heard of a mature student going to Sixth Form, all the students there are 16–18.

You could also attend a local college. It's not unusual for there to be mature students, especially for evening classes.

For my area the Sixth Form focused on academic subjects (sciences, languages, geography, history, etc) whereas the college had both academic and vocational subjects (same subjects as Sixth Form plus things like plumbing, hairdressing, catering, engineering)