Oki doki :). Probably not as much to fix as you think. I kept your basic proportions and such--most of what needed to be done was just little adjustments here and there.
Skull man: The hand is a bit small, and you've left out one of the joints in the thumb. I included a bigger drawing of a fist drawn realistically. I'm sure you could get it down if you did a few studies (just have a friend make a fist for ya). It’s harder to draw a skeleton hand making a fist, but if you get the basic gesture down you should be able to translate it into just bones :). It’s already so stylized that I don’t think you need to agonize over drawing all the bones (skeletal hands are very ambitious any way it goes though).
The shirt would wrinkle a bit where his arm connects to his shoulder.
The bottom hem should be at an angle as the raised arm would pull the fabric up a bit.
His knees are too high--I marked off approximately where they should be. Even though the figure is exaggerated, the legs are still a little short and the groin is a little too far up in comparison to everything else. The shoes should be more visible (band pants aren't wide enough to cover a shoe all the way) and the hem of the pant legs should arch a bit over the tops.
You might want to look at some real human skulls for basic anatomical information. You would probably benefit a lot from looking at Mexican dia de los muertos/day of the dead paintings/drawings/sculpture as well. The way you stylized the skull reminds me a lot of some of that art 
Center guy: I think you may be overcompensating for the shortened figure here, everything is moved up--most notably the armpits, shirt hem, and groin. The arms and to a lesser degree legs are also too short (the tips of his fingers should be closer to the middle of his thighs). The same shoe/pants hem issue comes up here as with Skull man.
I marked off where his knees are here as well, just to show why the arms and legs needed to be elongated
Ballet gal: Her head is turned, but her body is relatively forward oriented so there should be pretty much a straight line (following her spine) from the center of her collar bones, to the point between her breasts, to her belly button, to the center of her groin. Her collar bones were the only point on this line which were really off. I made a little bust up top to show approximately how everything would line up. Remember the muscle in her neck also. I made it fairly large in my drawing to illustrate how it might look when flexed, but since she's a smallish girl it probably wouldn't protrude as much.
Just a little note--you made the collar bones connect at the tops of the shoulders, which is really accurate. It's like my biggest pet peeve when people make collar bones grow out of the middle of shoulders or become like cancerous lumps right above the breasts xD. Keep that up 
Anyway, the arms aren't bad length-wise, just a little noodly. Remember the tendon which connects the arm to the body and forms the shape of the armpit, and the elbow joint. The fingers are well-placed (very graceful :)) but lack structure. Just remember the bones and proportions of the joints and you're golden.
Her breasts begin a bit too high, and her knees start too low. I drew in the basic shape of her body to try to help with that (it's always easier to start with a "naked" figure--it obviously wouldn't need any detailing--and then add clothes on top). It's easiest to reference the placement of knees and breasts by where the torso ends. Since you aren't doing realistic anatomy you can't use head lengths which makes it more difficult. In this case (since her torso is so shortened) it's probably easiest to think of the bottom of the groin as a mid-point between the breasts and the knees.
Her legs weren't that badly done other than the knee placement. The calves and feet are just a bit too small. Remember to give things a structure under the skin. The calves need a bit of muscle and fat, and the feet need heels. I looked up some pictures of pointe dancer feet and did a sketch beside to show what I mean a little. I wasn't sure exactly how you wanted the feet turned so I just made them a less awkward size on the sketch and showed two different orientations to the right. Remember to give her heel bones--that's the most important thing in this case I think
I would recommend learning how muscles and bones work at least a little bit, and how figures are actually proportioned. This kind of stylization is mostly about eyeballing, unfortunately, which takes a good knowledge of how things actually fit together. Do some life studies (it's best to work from real people, but photos can also work if you can't get into a figure drawing class or get your friends to sit still, hehe) of people with and without clothing on and get some good books on the subject. My favorite figure drawing books are by Giovanni Civardi. Don't use books like How to Draw Manga for anatomy. Those books are great for learning to draw anime faces and monsters and such, but if you use them for anatomy you'll pick up a lot of bad habits which will hurt you when you try to stylize figures like this
Basically, the figure is layers: bones form the structure under muscles and tendons which form the structure under skin which dictates where wrinkles are in whatever clothing goes over the top. Don't forget that and you'll be fine
Hope that was helpful. I know I talked a lot, haha
I tried to redraw the hands... but that just didn't work out the way I wanted... 
1) becuase I ran out of room on the paper, and 2, becuase I just couldn't get them to look right :\
