senior vs sr.

senior is a noun but sr. is not a noun.

Word	Noun	Adjective	Verb	Adverb
senior	Yes	Yes	No	No
sr.	No	Yes	No	No

senior (noun)	sr. (noun)
an undergraduate student during the year preceding graduation
a person who is older than you are

senior (adjective)	sr. (adjective)
older; higher in rank; longer in length of tenure or service	used of the older of two persons of the same name especially used to distinguish a father from his son
used of the fourth and final year in United States high school or college
advanced in years; (`aged' is pronounced as two syllables)