Highschool or High school? Which is correct?

High school is the correct spelling. You should always use two words to spell high school. Highschool is incorrect and should not be used in English. 

I went to high school for 7 years.

Do you see that the high school down the road was on fire?

High School Meaning

High School is a second-level school usually for children aged between 11 and 18. “High School” is usually used in North America and “Secondary School” is usually used in Europe. 

Is High School one word?

No, High School should be two words. 

Is High School hyphenated?

There is no need to put a hyphen in high school when we use high school as a noun.

High-school is correct when we want to use the adjective form. Remember we use an adjective to describe a noun and that noun usually follows the adjective.

Examples of hyphenated adjective

High-school students

High-school report cards

High-school yearbook