Joe the plumber was none of the above. He was an evidentiary example. A data point. The principal potential fallacy is a fallacy of composition,the assumption that Joe the plumber is a representative member of a larger class of individuals who would be so injured. But demonstrating that it is a fallacy requires demonstrating that others would not be so injured.
You are correct. If the teacher knew what he/she was doing, I think that this would be an excellent exercise.