underclassman vs junior

underclassman and junior both are nouns.

Word	Noun	Adjective	Verb	Adverb
underclassman	Yes	No	No	No
junior	Yes	Yes	No	No

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

Other hyponyms of underclassman include fresher, freshman, soph, sophomore.

underclassman (noun)	junior (noun)
an undergraduate who is not yet a senior	the younger of two persons
	a third-year undergraduate
	term of address for a disrespectful and annoying male

underclassman (adjective)	junior (adjective)
	younger; lower in rank; shorter in length of tenure or service
	used of the third or next to final year in United States high school or college
	including or intended for youthful persons