underachiever vs pupil

underachiever and pupil both are nouns.

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

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

underachiever: a student who does not perform as well as expected or as well as the IQ indicates
pupil: a learner who is enrolled in an educational institution

Other hypernyms of underachiever include educatee, student.

underachiever (noun)	pupil (noun)
a student who does not perform as well as expected or as well as the IQ indicates	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)