The rest of the story is pretty mind-blowing. I mean - in my day, you had to attend a meeting with the school head with your parents and have an interview before you were accepted as a pupil.
There would also be a paper trail between your new school and the one you'd left (detailing reasons behind the change).
If I remember right (and I'm just out of Sixth Form, about to start Uni), secondary school ends at year 11 - I was 16 when I left. So I guess Yr. 11 is about 15/16, which would make Yr. 10 14/15.
I know Year 9 is also about 13/14 because I remember having a crush on someone who had gone back a year and was thus a year older - and they were 14 at the time.