This article considers the components necessary to address the question, are single-sex schools more effective at whole-school wellbeing and mental health promotion than co-educational schools? The apparent lack of literature in the field raises the need for studies that use a generic indicator of whole-school wellbeing promotion across diverse settings. The recent development of a brief school-based instrument designed for use in educational practice and in research, is presented as a viable generic indicator of whole-school wellbeing promotion. The Survey of School Promotion of Emotional and Social Health (SSPESH) differentiates states of high, moderate, and low implementation of whole-school policies and practices that promote the emotional and social health of students.