Purpose: Whole-school approaches emphasising pupil participation are recognised as being conducive for building social capital, yet how participatory health educational processes relate to different types of social capital remains unclear. The purpose of this paper is to explore which mechanisms within a participatory health educational process influence social capital and collective actions in the school context, and to discuss children's agency in such processes. Design/methodology/approach: A multiple case study design, with the Danish "We Act -- Together for Health" intervention, considered as an instrumental case regarding participatory health educational processes for children, principally since it applied the participatory Investigation--Vision--Action--Change (IVAC) methodology. The paper is based on a theory-driven, abductive research strategy. Qualitative methods, including focus group interviews with children, semi-structured interviews with teachers and school principals, and participant observation were used. Findings: The study's conceptual framework, which elucidates several mechanisms that interact with types of social capital and collective actions within the school setting, indicates that working with child participation through the IVAC methodology can influence types of social capital and collective actions. It also emphasises children's limited agency in terms of affecting bridging and linking social capital, norms of reciprocity and collective actions without sufficient support mechanisms at the school and class levels. Originality/value: The study provides a novel comprehensive conceptual framework identifying the specific mechanisms at different levels that influence social capital and collective actions.