The aim of this study was to investigate factors that can predict collective efficacy in student work groups year 5 and 8 at compulsory school and to see if there are gender and year differences for efficacy beliefs and aspects of interdependence. A total of 283 completed questionnaires were analysed. Hierarchical multiple regression was used to predict collective efficacy and 2 × 2 ANOVA was used to analyse gender and year differences and interactions for following five factors: collective efficacy, self-efficacy, negative interdependence, positive interdependence and importance of good assessment and marks. The result showed that independent of gender, year and school, self-efficacy, positive and negative interdependence predicted collective efficacy in connection with group work assessment. The result also showed that there were better conditions for cooperation in year 5 compared to year 8. Additionally, it was significantly more important for girls than boys to achieve good assessment and marks.