My figure drawing class in college lasted three hours, three times a week.  This is to explain that the skill of working from a live model takes time and drill. For high school classes, each period for a couple of weeks should be used in order for students to gain a footing with the technical skills needed. In my college classes, students warmed up with gesture drawings, then did quick ten  to fifteen minute studies, all afternoon. 

I have observed that people outside the discipline do not understand this approach. Students assume they must develop a completely realized image with accurate rendering of all details each time they draw.  Consider the examples of student work below..  The most significant quality in each drawing is accurate measurements.  The surface rendering is not important because the proportions of the major shapes are the most essential skill to gain first, before details are considered.  The rendering of the surface detail comes later.