junior vs underclassman

junior and underclassman both are nouns.

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

As nouns, underclassman is a hypernym of junior; that is, underclassman is a word with a broader meaning than junior:

Other hypernyms of junior include lowerclassman.

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

junior (adjective)	underclassman (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