This study investigated whether an intervention, in the form of short educational sessions, influenced pupils' experiences of group work or cooperative learning (CL). The hypothesis tested was that an intervention for teachers and pupils would lead to pupils' increased (a) collective efficacy, (b) self-efficacy and, (c) positive interdependence, as well as (d) less negative interdependence. The participants were pupils from years 5 and 8 in three compulsory schools in Sweden, working in 22 groups divided into one intervention group and one control group (11 work groups in each condition). Data were collected through a questionnaire before and after participation in the study and analysed using a repeated measure ANOVA and 2×2 ANOVA. The results showed an increased collective efficacy, self-efficacy and positive interdependence and a reduction of negative interdependence. The conclusion is that the intervention provided for teachers and pupils did have an effect, thus promoting successful working as a group.