Background: Interventions are needed to improve health behaviors among Moroccan adolescents. Obtaining an understanding of suggestions and preferences of the target population can help develop such interventions. Purpose: The aim of the present study was twofold: (1) to explore social-ecological suggestions of participants for improving physical activity (PA), dietary behaviors and for reducing substance use among adolescents, and (2) to identify their preferences regarding delivery mode, facilitators and content for a school-based intervention. Methods: Seventeen focus groups were conducted with 100 adolescents, parents and teachers. Discussions with participants were mapped using a semi-structured guide, which was underpinned by the social-ecological model as the theoretical framework. Data were analyzed using a thematic analysis. Results: Four themes were about social-ecological (i.e. social, physical, distal and individual environment levels) suggestions on how to improve adolescents' health behaviors on PA, dietary behaviors and substance use. Three themes were about participants' preferences regarding delivery mode, content and facilitator of a school-based intervention. Discussion: A comprehensive approach was perceived as effective to improve adolescents' health behaviors. Participants' preferences on a school-based program are useful to develop such a program. Translation to Health Education Practice: Health educators can use the findings to develop school- or community-based interventions.