Doesn't help if secondary school starts at a different age!
Two years after the begininng of secondary school; in general lower-secondary starts around age 11, and in the US it usually begins with Junior High which is often, but not always, 7th grade,
In the UK ages 12-13 are typically Year 7 of secondary school.
Year 7's regular age range is 11-12: you're 11 going into the September that the school year starts and will be 12 by the end of the following August.
So it is, oops miscounted. Never was any good at math.