Observational Drawing is probably the greatest challenge for students with visual impairments. If they can't really see what they are supposed to draw, how are they supposed to draw it? And it is especially tricky when the visual illusion of a 3D form is so different from the form itself. Take a cube, which is made up of six squares each with four right angles, but is drawn using three diamonds-no right angles. A cylinder is made with a circle on each end, but the illusion of cylinder requires the only circle included, to be replaced with an eclipse.
For those who are completely blind, it became more of a lesson in following directions and learning about how sighted people see the three dimensional world with illusions rather than necessarily truth. They could measure size relations between objects with their hands and then try to replicate that with wikisticks. Then it was a matter of explaining light sources and have them draw with charcoal on one side and smudging on the other.
My students may not become wealthy artists years from now, but they will be able to tell you the difference between a shape and a form (even many adults make the mistake of saying "3D shapes"), that the medium of charcoal is essentially burnt wood, and explain what value is and why it is important. Mission accomplished.