Well, then... I think the form of the exam should reflect the aims that you set when you began teaching them, i.e. if you wanted to teach them to discuss a certain number of subjects spontaneously then you can make the exam in the form of a discussion or dialogues. If you mainly developed their ability to make monologues, then it would be logical to give them a text and ask them to give an outline of it and discuss it.
I wonder what you have decided on ;)