Short term experimental studies by DiPerna and colleagues have demonstrated the efficacy of the Social Skills Improvement System (SSIS) to measure and improve the social skills and academic functioning of students from Preparatory year to Grade 3 and to decrease problem behaviours. This longitudinal school-led project applied the multi-tiered assessment and intervention components of the SSIS to achieve sustainable outcomes. In each of three project years up to 16 teachers completed the SSIS Performance Screening Guide for approximately 380 students, with nearly half having special needs. Students screened by their teachers as having low levels of prosocial behaviour were assessed using the comprehensive SSIS Rating Scale. The collective data informed the social skills goal priorities among 10 skills targeted and taught by classroom teachers using the SSIS Classwide Intervention Programme (CIP). Intervention effectiveness indicators included change in prosocial behaviour, academic achievement, problem behaviours, wellbeing, and attendance. Students showed marked improvement on all outcome indicators across the three project years, resulting in an improved school climate. Limitations to the evaluation are discussed, followed by recommendations for other schools to adopt similar interventions to achieve sustainable improvement in students' social skills and other outcomes, and school climate.