girls' school
Refers to a school exclusively for female students, typically from junior high through high school, though sometimes including elementary or university levels.
She attends a local girls' school.
There are several well-known girls' schools in this area.
女子校 is a girls-only school, the female-side counterpart to 男子校 and likewise the opposite of 男女共学.
student of a girls' school
Compound of 女子 (joshi, 'girl, female') and 校 (kō, 'school').