The Diagram Exercisewritten 2000
Because of the nature of the interaction between the student giving the instructions and the rest of the class, assessment of this exercise is hardly required. The instructor has the burden of responsibility to do as good a job as possible so that the rest of the students can produce a correct drawing and so "beat the teacher" who specifies the diagram from the outset. However if some kind of assessment/reward is required, the teacher can select the pictures that most closely resemble the original picture.
The exercise can be repeated regularly over a period of time so that each student gets a chance at the "instructor" role. As the exercise is repeated over time the quality of communication will improve dramatically. The teacher may become more specific about the accuracy required of the pictures produced and may make the images more complex, but not too complicated, so as not to take up too much class time.
If the students know that this is going to be a regular exercise, they will be thinking in the meantime of how they will best go about describing the picture when it is their turn.