Here's a recent self portrait done on scratchboard. I gave this same assignment to my students at the Governor's School for the Arts, and told them I would do the same project with them. A lot of my students had never done scratchboard work before so they were very intimidated. I got them to break the ice and overcome their "deer in the headlights stare" by doing several drawings on black paper with white chalk to get comfortable with the concept of "drawing the light". The first drawings they did were a little shaky, but I could tell the concept began to sink in. After that, I gave them a demo on how to light a portrait for reference, and then how to scratch into the clay board with different tools for a variety of marks and effects.
I remember in my Art classes I always wanted to see what my instructors/professors work looked like. If I had a figure drawing teacher, I wanted to see what their drawings looked like...painting teacher, I wanted to see their paintings, etc. So I definitely didn't want to negatively affect the confidence of my students by participating in the assignment, but rather teach them by "doing". I also told them up front that if they didn't want me to participate, I wouldn't, but they wound up doing an incredible job on the assignment and they all told me they liked that I did the project with them.