Posture Counts

I'm planning on collecting these sketches of violin students and giving them to my son's violin teacher as a gift. I'm thoughtful that way. I'll admit I'm a little resentful of the pressure of having to do a good job when I'm sketching these, knowing they'll belong to someone else! Ironically, our teacher spends most of her time correcting bad posture, and I find the most interesting poses to draw, (naturally) demonstrate atrocious posture. I was quite pleased with the wrinkles in the clothing on the figure on the right .... how do I do it?!