pupil vs schoolboy

pupil and schoolboy both are nouns.

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

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

Other hyponyms of pupil include boarder, day boarder, latchkey child.

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