sister school
Refers to a school that has a formal partnership or exchange relationship with another school, similar to 'sister city'.
Our school is a sister school with a high school in Australia.
Students from the sister school are coming to Japan next week.
Compound of 姉妹 (shimai, 'sisters') and 校 (kō, 'school'), modeled after terms like 姉妹都市 (sister city).