Secondary schools tend to sponsor a large number of extra-curricular activities (ECA) yet little is known about their contribution to students' educational outcomes. This meta-analysis aims to determine what it is about ECA participation that supports positive educational outcomes. Furthermore, this study challenges the theoretical assumptions about the benefits of participation in ECA. 29 studies (all except for one based on data collected in the United States) met the search criteria for inclusion in the analysis. Most effect sizes on academic achievements yielded from non-specific ECA, academic clubs and journalism were small, as were participation in performing arts, sports and leadership activities on a range of educational outcomes. Although the results show associations between participation in ECA and educational outcomes, causal effects could not be confirmed. It is concluded that the lack of evidence supporting the causal effects, and thus the common theoretical assumptions about the effects of ECA on educational outcomes, is due to methodology limitations in these studies.