Purpose: The purpose of this paper is to address school-NGO interactions by analyzing the power of foundations -- a specific type of third sector organization or NGO in education. Design/methodology/approach: Data are collected through a quantitative survey, qualitative interviews, official documents, reports and websites. Social network analysis and grounded theory are used to analyze the data with the aim to develop a theoretical approach. Findings: The study identifies three dimensions, i.e. relational, structural and discursive dimensions of power. Based on the analysis of an illustrative multi-stakeholder initiative, the paper highlights the role of foundations in framing educational settings, concepts and structures of the education system as such. Practical implications: The three-dimensional power perspective offered in this paper is particularly useful for scholars investigating school-NGO interactions or multi-stakeholder partnerships in education. Furthermore, it is of crucial importance for practitioners, school principals and education administrators dealing with school-NGO interactions given that foundations seem to be increasingly able to draw on new sources of power in these interactions. Originality/value: While the number and power of the third sector in education continues to rise worldwide, there is wide consensus that NGO power in education has, to date, hardly been researched. This paper contributes to this dearth of research by uncovering foundations' different sources of power and by developing a theoretical approach for analyzing the power of third-sector organizations in education.