youth vs pupil

youth and pupil both are nouns.

Word	Noun	Adjective	Verb	Adverb
youth	Yes	No	No	No
pupil	Yes	No	No	No

As nouns, pupil is a hyponym of youth; that is, pupil is a word with a more specific, narrower meaning than youth:

Other hyponyms of youth include blade, hobbledehoy, pup, puppy, school-age child, schoolchild, slip.

youth (noun)	pupil (noun)
the freshness and vitality characteristic of a young person	the contractile aperture in the center of the iris of the eye; resembles a large black dot
young people collectively	a learner who is enrolled in an educational institution
a young person (especially a young man or boy)	a young person attending school (up through senior high school)
early maturity; the state of being young or immature or inexperienced
the time of life between childhood and maturity
an early period of development