Co-operative group learning is known to be an effective instructional practice, especially for mathematics. While group work can be incorporated into the classroom in multiple ways, not every group is a co-operative learning community. In the current study, class observations and preservice teacher interviews were used to compare experiences during formal and informal group work. The findings indicate that formal co-operative learning groups, or prolonged interaction with the same group of people, develops a sense of strong community within the classroom, thereby providing a safe space and facilitating discussions. Preservice teachers involved in formal co-operative learning reported they learned better, gained confidence in the subject, and associated positively to using group work in their future classroom. However, preservice teachers involved in informal co-operative learning had neutral perceptions about group work. We suggest that prolonged interactions among the group members is required to establish effective co-operative learning groups.