junior vs junior-grade

junior is a noun but junior-grade is not a noun.

Word	Noun	Adjective	Verb	Adverb
junior	Yes	Yes	No	No
junior-grade	No	Yes	No	No

junior (noun)	junior-grade (noun)
the younger of two persons
a third-year undergraduate
term of address for a disrespectful and annoying male

junior (adjective)	junior-grade (adjective)
younger; lower in rank; shorter in length of tenure or service	inferior in rank or status
used of the third or next to final year in United States high school or college
including or intended for youthful persons