sixth-former vs pupil

sixth-former and pupil both are nouns.

Word	Noun	Adjective	Verb	Adverb
sixth-former	Yes	No	No	No
pupil	Yes	No	No	No

As nouns, pupil is a hypernym of sixth-former; that is, pupil is a word with a broader meaning than sixth-former:

Other hypernyms of sixth-former include educatee, student.

sixth-former (noun)	pupil (noun)
a student in the sixth form	the contractile aperture in the center of the iris of the eye; resembles a large black dot
	a learner who is enrolled in an educational institution
	a young person attending school (up through senior high school)