senior high vs highschool

senior high and highschool both are nouns.

Word	Noun	Adjective	Verb	Adverb
senior high	Yes	No	No	No
highschool	Yes	No	No	No

As nouns, senior high and highschool are synonyms defined as:

senior high and highschool: a public secondary school usually including grades 9 through 12

Other synonyms of senior high include high, high school, senior high school.

senior high (noun)	highschool (noun)
a public secondary school usually including grades 9 through 12	a public secondary school usually including grades 9 through 12