boys' high school
Refers to a high school that only enrolls male students.
He attends a local boys' high school.
女子高校 is a girls' high school, the female counterpart to 男子高校.
共学 refers to coeducation; a 共学校 is a coeducational school, unlike a single-sex 男子高校.
Compound of 男子 (だんし, 'boy, male') and 高校 (こうこう, 'high school').