senior high school vs highschool

senior high school and highschool both are nouns.

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

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

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

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

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