junior school vs primary school

junior school and primary school both are nouns.

Word	Noun	Adjective	Verb	Adverb
junior school	Yes	No	No	No
primary school	Yes	No	No	No

As nouns, primary school is a hypernym of junior school; that is, primary school is a word with a broader meaning than junior school:

junior school: British school for children aged 7-11
primary school: a school for young children; usually the first 6 or 8 grades

Other hypernyms of junior school include elementary school, grade school, grammar school.

junior school (noun)	primary school (noun)
British school for children aged 7-11	a school for young children; usually the first 6 or 8 grades