spring chicken vs pupil

spring chicken and pupil both are nouns.

Word	Noun	Adjective	Verb	Adverb
spring chicken	Yes	No	No	No
pupil	Yes	No	No	No

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

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

spring chicken (noun)	pupil (noun)
a young chicken having tender meat	the contractile aperture in the center of the iris of the eye; resembles a large black dot
a young person (especially a young man or boy)	a learner who is enrolled in an educational institution
	a young person attending school (up through senior high school)