Each class has one or more student groups that attends the class. In this example we have a group for all BDS5 students, other groups for A,B,C,D. Finally we have subgroups A1 and A2 which are subgroups of A. There are 2 subgroups of each group B,C,D also.
Therefore each student can belong to more than one group.

What we really need is to be able to define the student groups in a department. Either manually or importing a .csv file with Student ID, Group ID
Where this could really help is when enrolling students to classes. Currently you can either import a .csv of students or manually add the students to the class.
I propose a third way; where you can simply pick one or more student groups that will attend that class.