Shaping a Skeleton (and a classmate)

The first thing we were told to do in this lesson was to draw the torso of a skeleton which was placed in the center of the room in ten minutes, the result I got for this first drawing wasn't as good as  I hoped as I went too big and was focusing on getting the rib cage right. We were told to do it again but this time I had made it smaller so I could have more time to draw the whole thing. For the size that it was, it turned out pretty good.
Next we were told to make a 2D shape 3D with grid lines, So I experimented on a random shape first, then continued by drawing parts of the skeleton. The one i focused on the most was the Skull, which i am quite proud of. However I think I need to work on the proportions of the skull. Skeletons have always been a weakness of mine to draw. I Experimented with the pelvis and the ribs, but to me they don't look like much, Probably if I had added the spine to them, they would look more like their intended target,
Lastly we were told to draw one of our classmates and I was partnered with a man named Aaron, I believe I had an hour and a half to draw him and below was the result I made.
Another successful class.

Two Torsos, both took 10 minutes

Practicing with grid lines, left clockwise: Round Shape, Skull, Ribs and Pelvis 

Sketch of My classmate Aaron, as he was drawing me