youth vs schoolchild

youth and schoolchild both are nouns.

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

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

youth: a young person (especially a young man or boy)
schoolchild: a young person attending school (up through senior high school)

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

youth (noun)	schoolchild (noun)
the freshness and vitality characteristic of a young person	a young person attending school (up through senior high school)
young people collectively
a young person (especially a young man or boy)
early maturity; the state of being young or immature or inexperienced
the time of life between childhood and maturity
an early period of development