pupil vs schoolchild

pupil and schoolchild both are nouns.

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

As nouns, pupil and schoolchild are synonyms defined as:

pupil and schoolchild: a young person attending school (up through senior high school)

Other synonyms of pupil include school-age child.

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