Intervention Activities and Implementation Strategies for School-Based Health Promotion: Identifying Core Functions and Forms to Facilitate Scale-up of an Effective Intervention

<b>Background:</b> School-based health promotion is a key public health strategy to reduce disease burden and health inequalities. School-based interventions with local evidence of effectiveness need to be scaled up to maximize their benefits. A Project Promoting healthy Living for Everyone in Schools (APPLE Schools) is a health promoting school (HPS) intervention that targets schools in disadvantaged settings and has been shown to be effective in promoting children's healthy lifestyle behaviors and reducing health inequalities. To support its scale-up, we aimed to identify core functions (basic purposes driving intervention's effectiveness) and forms (specific content and delivery strategies implemented to achieve core functions). <b>Method:</b> We extracted 5,301 action items from 191 annual action plans written between 2011 and 2021 in 70 APPLE Schools. We followed an implementation science approach and used supervised machine learning algorithms to classify 2,683 unique action items into intervention activities and implementation strategies. Core functions were drawn from theoretical frameworks; forms were identified through thematic analysis. <b>Results:</b> We identified 55 forms and mapped them to 17 core functions of intervention activities and implementation strategies. The most common core functions of intervention activities were enablement (96%), modeling (66%), and education (54%); the most common core functions of implementation strategies were relational and organizational support context (86%), partnerships and networking (84%), student participation (78%), and professional development and learning (73%). The remaining core functions were identified in <50% of the schools. Forms included a broad range of activities, with a greater variety of those that addressed the most common core functions. <b>Conclusions:</b> We created matrices of core functions and forms of intervention activities and implementation strategies to inform the successful scale-up of APPLE Schools, an effective and cost-effective HPS intervention. These matrices can be used as a guide to improving existing HPS interventions and scaling them up to new settings.