sophomore vs second-year

sophomore is a noun but second-year is not a noun.

Word	Noun	Adjective	Verb	Adverb
sophomore	Yes	Yes	No	No
second-year	No	Yes	No	No

As adjectives, sophomore and second-year are synonyms defined as:

sophomore and second-year: used of the second year in United States high school or college

sophomore (noun)	second-year (noun)
a second-year undergraduate

sophomore (adjective)	second-year (adjective)
used of the second year in United States high school or college	used of the second year in United States high school or college