A student may attend multiple subjects within the the Group (of students) but each student may belong only to one main Group.

Apart from the main Group , students may also belong to multiple other Groups.
Lets say that 4 students are in the main Group Group 7 Blue

Student A - Main Group : 7Blue
Student B - Main Group : 7Green
Student C - Main Group : 7Blue
Student D - Main Group : 7Green

Now lets say that Student A and B select Greek Native 7 and C and D select Greek Beginners 7
Let's say on Wed Period 3 all students attend 1 hours Greek class.

In this case student A and B is one group of students attending Greek Native 7 (So the Group is called also Greek Native 7 or whatever name you want to call it) . The Group Greek Native 7 attedns the subject Greek Native 7 in e.g. Classroom E1 , every Wed , period 3
Studet C and D , is a another group of students. Their group is called Greek Beginners 7 (or whatever else) and they are in this group every Wed , period 3, in classroom E2 attending the subject Greek Beginners 7

If however you had 55 students attending Greek Native 7 then you probably need 3 groups
So you will have
Greek Native 7 A --- Subject: Greek Native 7 - with 15 students - Classroom E4 - Teacher - Maria
Greek Native 7 B --- Subject: Greek Native 7 - with 20 students - Classroom E5 - Teacher : Petros
Greek Native 7 C --- Subject: Greek Native 7 - with 20 students - Classrrom E3 - Teacher : Nikos

For the time being for example, we can automatically create only one group available for all selective subjects and give autoatmically the same name as the subject

You may:
- Want to change the name of theese groups
- create extra groups if a lot of students attend any of these selective subjects.