Katakana noun meaning a high school, especially with a foreign or English-language feel. For ordinary Japanese schools, 高校 or 高等学校 is usually the more standard wording.
彼女はアメリカのハイスクールに通っている。
She goes to a high school in the United States.
The movie is set in a high school.
She attends a high school in America.
He graduated from high school last year.