I think you could be a little bolder in the initial lay in, for instance marking the darkest dark and lightest light right away. I find it easier to get my values right when I have the whole range down to compare to, but I guess that's personal preference.
I can see that your soft edges are a recurring problem, so I'll try to give a couple tips on that (I'm a big believer that if you find a chronic problem, you should hit it dead on, so I hope this doesn't come across as harsh!)
I think that you are using brushes that are too soft and/or too low of opacity for your current needs. Those brushes have their place, but I think they are exacerbating your problem. A lot of people when relatively new to digital painting seem to gravitate towards airbrushes and other squishy brushes (including me), and I never really figured out why.
A suggestion: Try doing a few more studies (any study, doesn't have be one of these studies) with 100% or near 100% opacity, like you did in #11. Use a hard round brush so your edges are razor sharp. Try and get those hard edges ingrained into your mind, and get used to your strokes looking harsh at times, before you go back to doing the soft stuff. It may also help you when blocking in shapes because it forces you to kind of fill things in, like a paint-by-number, and you have to carry each shape until it's very edge instead of building up the middle and letting the edges kind of disappear. It might look terrible at first but that doesn't matter, what matters is that you are physically unable to create soft edges, and you are forced to be bold.
Also, the sharpen filter can be a helpful tool, but I wouldn't rely on it too much. Better to learn how to do the sharp edges yourself, and then use the filter for quick and handy fixes once you have the basics down.
Anyway, not sure I described that well, but hope it helps. You're improving a lot already, can't wait to see the rest of your studies!