sixth form 
noun British
The two final years at school for students between the ages of 16 and 18 who are preparing for A or AS levels.I am very relieved for her sake that there is still time for the new sixth form curriculum to be sorted out....
- I used to know a teacher who took a couple of her sixth form students out in the car at lunchtime for a sly cigarette.
- Now, reports the Times & Citizen, a sixth form student was jailed for six years on Monday.
Derivatives
sixth-former
/ noun ...- ‘My life was completely ripped apart on that night in May,’ said the former sixth-former at Gumley Convent School.
- The sixth-formers at Allerton High School, Leeds, say their university careers could be threatened because of a printing error in a further maths AS level set by the exam board
- Judy Turner, leading the research at Manchester University, has been conducting psychometric tests on hundreds of university students and sixth-formers at local schools and colleges.