A sister school is usually a pair of schools, usually single-sex schools, one with female students and the other with male students. This relationship is seen to benefit both schools. For instance, when Harvard University was a male-only school, Radcliffe College was its sister school. The sister school concept as a single-sex school began to change as several institutions adopted coeducational environments starting in the 1970s due to the increasing awareness or consciousness about sex bias and discrimination.
Background
The term sister school (or brother school) has several alternate meanings:
two schools that have a strong historical connection
two schools which have social activities involving students from both schools
two schools under the same management
two schools built using the same floor plan/layout
two schools in different nations that have established a collaborative international partnership.
Examples of Sister Schools (United States)
The Seven Sisters (colleges) are a group of originally seven (now five) private liberal arts colleges that are in the Northeastern United States. The majority of them were chartered between the 1860s-1880s.
↑Datnow, Amanda; Hubbard, Lea (2013). Gender in Policy and Practice: Perspectives on Single Sex and Coeducational Schooling. London: Routledge. p.197. ISBN9781136703775.