Yeah, but high school starts at year 9 (age 13-14), so:
(Y9 doesn't exist for a bit)
Y9 --> Y10
Y10 --> Y11
Y11 --> (Either leave school or just take a few extra weeks off)
Y12 --> Y13
Y13 --> (Leave school*)
This is done because, after the exams, the Y11s and Y13s have no content left to learn, so there's no point in keeping them at school.
Also, as I said, my school is strange for doing this. Most, if not all, other British high schools are normal.
*Unless you get held back, stay on for another year, or go to university