We had Grades 1 and 2, then standards 1 to 10. Form 1 was standard 6 - the start of High School, through to Standard 10 (Fifth form, "A" levels second year, Standard 9/fourth form being "A" first year).
If you reach 16 in 5th form then 13 years at school would mean you started school at 3 years old.You are right, sort of. But its 13 years at school which still makes it 4th form in old money.
They only do 12 over here. Then again when they finish at 18 they are at the same level as a kiddy that has done their GCSE's in the UK. For A Level Equivalent they either have to do college level courses at school, or they go to college after they finish school. Which is why their degrees are 4 years in length and not 3 years like the UK - the first 2 years gets them up to A Level standard.
Children in the UK have to legally attend primary and secondary education which runs from about 5 years old until the student is 16 years old.