Beautiful work. I really love the sharp contrasts in your shading, and you captured the essence of each pose really well considering the time you had for each (especially the 15 minute one!).
I loved all my figure drawing classes and wish there were more available to take around me right now. My biggest lesson I had to learn was to do a quick general gloss - over sketch just to capture the composition and figure really quick. I always have an inclination toward drawing the detailed stuff right off, which seemed to be a no-no.
Anyway, I love this and I hope you upload more of these sketches! I think these are great to have in the Portal and for others to see your process and practice work.