Groups can be united in many ways.
In school, groups find unity through shared activities and shared interests. We see this exemplified in clubs and sports teams.
Also in classrooms, group activities and group presentations can help create a sense of unity.
Generally we can say that groups are defined "positively" and "negatively". Discussing groups in this way may help us to see how group unity can effectively be generated.
When a group is defined positively, it is defined by what is present: the presence of shared interests, of commonality, etc. This connects with the notion of school clubs.
When a group is defined positively, it is defined by what "it is not". This is true of sports teams. Teams compete against groups that are distinctly in conflict with themselves. One team is not the other, by definition. They are defined by who they are not; defined by opposition. This is also true of group presentations. The presenting group is not part of the rest of the class while it offers its presentation.