It is late, and I had 12 hours long classes... sorry for being like spagetti here, and I do not take responsibility for saying anything not correct, this is only how I know it, not how it is the best:
Well, let me focus on the picture number one. First, I like how you draw hair, and that you make solid folds on clothing (often forgotten by people), what irks me is the anatomy.
Firstly, their heads are to big, there is a set ratio of how big the head in each age should be acourding to the body.
some info about the very basics Also the girls leg may be to short and thin, but I am not really shure. Anyways the hand of a person has a set size as well. If you put your hand on your face, than if the end of your palm is at your chin, than the end of your finger should reach the top of your fore head (may vary a little, and also the face can be roughly devided to 3 almost equal sections, one the forehead, one the nose, and one from the nosebottom to the chin. Also if looking foreward you can make to lines from the middle of the pupil, these lines give the ideal ends of your mouths width. And the earlobe is roughly in the middle of the head if you look sideways, wich means there is the same length to the left from it as to the right in about.). The girls back is awkward as well, I don't think it should bend in this way, try the pose out before a mirror, or ask a friend to model. Starting of with a very rough geometrical shape based sketch that you refine step by step, and practising of stock photos from deviant art, or poemaniacs.com will help you on these thing relativly fast. And even if you enjoy drawing anime, practise in realistic, and only do anime for fun till a while. Converting real people to anime is way easier than trying to achive good anime pics without a background, I know from experience, one year life drawing gave me everything I know about drawing (mostly).
On a side note, don't be afraid to put weight, muscle groups, and some fatt on your characters, a little chubby is better and more beliveble than to thin, and there is no such thing as a straight line in a body. EVERYTHING bends, even if almost not visibly, but it does. So no sharp knees, not even in doodles, it make you used to see it, wich is bad.