Most interventions designed to prevent HIV/STI/pregnancy risk behaviours in young people have multiple components based on psychosocial theories (e.g. social cognitive theory) dictating sets of mediating variables to influence to achieve desired changes in behaviours. Mediation analysis is a method for investigating the extent to which a variable X (e.g. intervention indicator) influences an outcome variable Y (e.g. unprotected sex) by first influencing an intermediate variable M (e.g. self-efficacy to use a condom) and provides a way for empirically validating theoretical hypothesised mediators. In this way, mediation analysis is a critical tool for suggesting which components of complex interventions should be the focus of more efficient and effective interventions in the future. The present study applied multilevel mediation analysis to outcome data from the All4You2! study to begin to examine the relationships between a theory-based HIV/STI/pregnancy prevention curriculum for students attending alternative high schools who were at risk of educational failure. The study targeted psychosocial mediating variables and the primary outcome unprotected sex in the past three months. Results suggest helping young people attending alternative schools identify and avoid exposure to risky situations and improving their self-efficacy to refuse sex should be focal points of future interventions.