It really depends on the lighting situation and the methods by different artist, and I'm sure your professor is very good at what he does. And certainly you shouldn't be put off by the fact they they don't like sci-fi or fantasy... to be honest, sci-fi/fantasy genre in general lacks the subtlety that the fine art market requires, so it is natural that some of them don't find them appealing.
Here is an example of a figure drawing that is close to what I have learned from my school. This is a figure drawing done by William Bouguereau. The white is used as highlights to bring out the form.
But here is a figure drawing done by Pierre-Paul Prud'hon, which uses white as light to separate the planes, and it works just as well. I think this might be what your professor was going for. Do notice how Prud'hon put his white down very carefully so there is no mixing.
It might appear that the first image's light source isn't as strong as the second one, but they could actually all had similar amout of light, except that white is used differently here. Prud'hon and Bouguereau has different styles, and their figure drawing reflects to their style. Overall, my point is that both methods will achieve amazing result if you really spend your time getting the core shadow, the form shadow, the cast shadow, and the proportions right.