Single-sex education maintains its presence in the school scene in Spain, in an environment of generalized co-education. Although there is some research on the reasons for maintaining or promoting this school model in Anglo-Saxon countries, there are no studies for Spain. The proposed objective is to delve into the reasons and motivations -pedagogical or others- of the creation or maintenance of the separation of the sexes in single-sex schools in Spain, and -in the perception of the directors- to know the results and good practices that out in their schools. It has been based on the qualitative analysis of twelve in-depth interviews with directors of these schools, which were recorded, transcribed and processed with the Atlas.ti program. The analysis of the results allowed us to reach various conclusions: separation is perceived as a practice that enhances the objectives of personalized education; differentiated schooling, within adequate educational planning, brings advantages for aspects such as gender equality or school excellence; it allows to respond to a demand of the families; the inclusion of training plans for equality in these schools is a general practice. Consideration of the educational significance of teachers as a gender model to achieve greater equality of opportunities is also highlighted.