highschool vs high school

highschool and high school both are nouns.

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

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

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

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

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