I do want to mention that when I did this with my students, we spent a class just practicing. We had different students be our models (one girl and one boy). I asked the students to observe the pleats in the girl's Mitz (bonnet), then the girls practiced drawing one. Similarly, the boys checked out the suspenders. These two items of clothing are of course what gave our portaits their Hutterite flavour. They all added background. I had the older students do a bit more on perspective than the younger ones did.
The most challenging part I found was drawing the hand holding the mirror. Ironically it turned out to be easiest for the students in grades 1 and 2. Since their portraits are about the same size as they are in real life, I simply had them hold their pencil as if they were holding a mirror handle and then traced around it. They added th details such as fingers and knuckles.