Starting out with the best parts first, I think you've got a good understanding of form and shape going on here. I really enjoy doing loose sketch work as well, and the shading works to the advantage of the piece.
There are some slight variations to each girl's pose, but part of me wishes there was a little more variety. They're all leaning in the same direction. Perhaps altering the composition so that the eye is drawn more than in just the one direction from right to left (or so it seems to me. The direction the eyes are looking tend to want to make me shoot in that direction, combined with the leaning in the same direction, it was a strong inclination). Seems a bit oddly cropped, cutting through the back on the girl on the left. Kinda almost makes a tangent with her back, making her seem to rest on the edge. If there is something she's resting on, may be better just to show whatever it is.
Aside from some occasional pinching at the ankles and wrists, good job at portraying anatomy as well.