prep school vs secondary school

prep school and secondary school both are nouns.

Word	Noun	Adjective	Verb	Adverb
prep school	Yes	No	No	No
secondary school	Yes	No	No	No

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

prep school: a private secondary school
secondary school: a school for students intermediate between elementary school and college; usually grades 9 to 12

Other hypernyms of prep school include Gymnasium, lycee, lyceum, middle school.

prep school (noun)	secondary school (noun)
a private secondary school	a school for students intermediate between elementary school and college; usually grades 9 to 12