Abstract Being assessed in group work is a balance between cooperation and competition. Self-efficacy and collective efficacy are important concepts in understanding how group work progresses and what attitudes assessment evokes. The aim of the study was to investigate the effects of a short educational intervention on the association between efficacy beliefs and attitudes towards being assessed in group work. In a randomized, controlled study of 22 pupil work groups, half of them got a short educational intervention. The work groups were formed for this study. The pupils answered a questionnaire before the intervention and after doing group work for 3 to 6 weeks with a study-specific task. A moderated mediation analysis showed that attitudes towards being assessed in group work significantly are related to self-efficacy mediated through perceived collective efficacy and that this relationship is stronger in the intervention group. In the context of work group assessment, we have shown that self-efficacy and collective efficacy are two separate, but related concepts that are dependent on each other when it comes to pupil attitudes towards group work assessment, and that a relatively short educational intervention to teachers and pupils had an effect on the attitude. However, the older girls’ attitude towards group work assessment was the least positive of all, which may indicate that the intervention depends on gender and age.