Java pupils enrollment system
<p>I have to design a small after school clubs management system with GUI (which has been designed at the beginnning). The system should perform a few functions:
- add a pupil,
- add a club,
- add a pupil to a specified club.
When user clicks on a club, it should list all of the pupils enrolled in the club. On the other hand, it should list all of the clubs the student is enrolled in when user clicks on a pupil. 
The first two functions are done and work properly. However, I'm struggling with adding a pupil to a specified club.</p>
