Hello there! I like the mixture of clothing styles, it's cool to see some variety. I have some advice, though.

Clothes will both be way more natural on more realistic body frames. All your characters have too much tension in their shoulders and are leaning forwards at the hip. Let them relax a bit. Other than than, the basic body looks good.
As for the hair, be careful not to pillow-shade it. Be bolder with your contrast, and really show off the properties of the character's hair beyond the simple stuff like its shape and color!
I made an edit for you, if it helps.



I didn't understand what the original hairstyle was. I made a new one that shows off what one type of hair looks like.
I love the variety, I think the base is good, and if you just give it that little bit of extra flair, you'll be there! Keep up the good work.

You're welcome!
To change the body frame, it's just a very simple matter of making the diagonal lines of the back and stomach areas fall straight, then (in this case) making the hips extend backwards and sideways from the body a little. You'll get it, don't worry.

Also: I went rooting around, and I found a few hair tutorials!

http://liiga.deviantart.com/art/Hair-Tutorial-46922701
-This one is simply awesome. As a side note, I think that most people wouldn't be angry if you only drew up to step 4. This tutorial goes above and beyond to show how to really add something to a character's hair.

http://red-priest-usada.deviantart.com/art/TUTORIAL-Drawing-Hair-142821893
-How does hair actually work? This gives some specific examples.

http://rosuuri.deviantart.com/art/Hair-Tutorial-397073984
-A more specific style, that provides a different perspective on the subject. There's way more than one way to do hair!

http://elsy123.deviantart.com/art/Hair-tutorial-308799748
-the actual material, hair, gets individual treatment here.

Hope these help!