I think your art is cute and has a nice style to it, but here are some issues:
1. Realistically, hands should cover the face. Put your hand up to your face; it should almost go from your chin to your forehead. If this is a style choice (since girl hands tend to be smaller and anime heads tend to be rather large) run with it.
2. For the bent arm, the upper arm is too long in comparison to the forearm. If you need a rule; the elbow should be at the waist and the fingertips should fall to mid-thigh. If you're going for smaller hands as mentioned in pt 1, then please adjust accordingly (IE don't make the forearm too long just 'cause I said your fingertips should fall mid-thigh)
3. There's too much space between her waist and her crotch. AKA her hips are too long. I don't actually have a good rule of thumb for this one, sorry!
4. Your legs (I forget if it's legs or if it's hip-down) should be half the length of your body. Of course, this is not really necessary as in anime style leg-length is often exaggerated (see especially: Clamp)
5. The curve of her left breast is too far to her left and also a strange curve that makes her breast seem smaller than the right one, as well as oddly pointy in the center.
6. Additionally, the X on her shirt doesn't really follow her breasts and adds to the fact that the left boob is smaller than the right boob.
7. Her left shoulder is too far in-- It illusions a nonexistent angle in her upper arm. If you extend the line from her back-line/armpit point, it should curve comfortably with her back and the top of her shoulder. Also, this shoulder should be wider than the right one anyway, since we're viewing her at a 3/4 angle.
8. Her feet are too small and I can tell from the shots you drew them that you're used to drawing them from the front or from the side! Try drawing them from different angles... naked feet, socked feet, shoe....d feet! It really helps. If you can't have a friend pose for you and you're not flexible enough to pose for yourself (let's face it, if you were you might be a contortionist instead) then try
http://posemaniacs.com; they have a foot in their flash... thingy.
9. Her left calf is concave on her left side. It should be convex. Actually, I find the shape of legs to be beautiful so I recommend looking at them~ Even your own, in the mirror. Legs are so nice.
10. In the big pic, her jaw doesn't seem to match up very well with the back of her head.
Things that are not really issues but may become so in the future, or generally just things that could be improved but are not really necessary:
1. Her right arm has no elbow. If this is a stylistic thing, go for it.
2. Her pinky and ring finger are kind of stiff.
3. There are too many clothing folds on her shirt, which makes it look too small.
Things you did nicely, so please don't change:
1. The overall thin/thick ratio. Her body works well with your style of art and the choice of clothing. Her arms are good thickness in relation to her waist; she has a good amount of curve for a teenager, etc.
2. Boobs! Though they could stand to maybe be a bit perkier, they are a good size for her general appearance.
3. Eyes. They're just well-drawn. Good size, good proportion, good foreshortening, and I love your style of eyeshine.
4. Her expressions. They're just adorable.
Anyway, I hope you found some of that helpful! Good luck and keep drawing ♥