friend vs classmate

friend and classmate both are nouns.

Word	Noun	Adjective	Verb	Adverb
friend	Yes	No	No	No
classmate	Yes	No	No	No

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

friend (noun)	classmate (noun)
a person you know well and regard with affection and trust	an acquaintance that you go to school with
a person with whom you are acquainted
an associate who provides cooperation or assistance
a person who backs a politician or a team etc.