School-based prevention and promotion interventions (SBPPI) improve desirable outcomes (e.g., commitment to school and attendance) and reduce undesirable outcomes (e.g., suspensions and violence). Unfortunately, our understanding of how to effectively implement and sustain SBPPI outside of well-controlled conditions is lacking. To bridge this science/"real world" practice gap, a system of sustainable implementation, which merges implementation strategies and sustainability strategies, is proposed for SBPPI. Ecological levels and phases affect this system. This conceptualization is supported by analyses from a diverse sample of 157 schools implementing Social-Emotional Character Development, a type of SBPPI. The system of sustainable implementation was measured using the Schools Implementing Towards Sustainability (SITS) scale, which was designed to be "user-friendly" in field settings by being viable and scalable. The SITS demonstrated strong reliability as well as promising concurrent and construct validity. Implications are discussed.