Preparing for pencil work

Students are getting ready to do self portraits, and I want them to use only ebony pencils to shade in a range of values.  We went over some basic shading practices, and then I also gave practice using grids to draw.  In my experience, students either love them or they hate them.  Either way, it is great to help them to draw only what they see.  I made some "Warholed" grey scale pictures with grids for the students to draw and practice making values with only their pencils. Here are the celebs I used:

When we are done, I plan to take students photos so they can draw themselves.