underclassman vs lowerclassman

underclassman and lowerclassman both are nouns.

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

As nouns, underclassman and lowerclassman are synonyms defined as:

underclassman and lowerclassman: an undergraduate who is not yet a senior

underclassman (noun)	lowerclassman (noun)
an undergraduate who is not yet a senior	an undergraduate who is not yet a senior