This article describes the methodology and program effects of the Secondary Schools Demonstration Project (SSDP) conducted in four Ontario schools. The objective of the study was to evaluate the extent to which a universal program model of three interventions--cooperative learning; classroom management; and peer-helping approaches that included tutoring and mediation--can reduce the prevalence of antisocial behaviour. The study employed a two-group matched comparison before-and-after design. It involved the collection of baseline data on all ninth grade students (average age of 14 years) in four schools, and the selection of a sub-sample for a more detailed follow-up. Of the 13 measures analyzed, the results were statistically significant on four variables: student perception of teacher, student sense of belonging, student attachment to school, and truancy, suspensions, and behaviour problems. The findings of the study are discussed in relation to the methodological issues involved in implementing and evaluating multi-component interventions in secondary schools. Secondary Schools Demonstration Project Outcome Measures are appended. (Contains 4 tables.)