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Man, i totally feel your pain: I went to art school too, and I just wish I'd had more live drawing and less arty- farty stuff... Anyway, let's get to things.
I've noticed your portraits are gradually coming together and looking convincing, but there's a tiny little things that I don't quite like: the eyes. They look a bit too big, like you're putting too much effort on them, trying to make them expressive. That's fine, as long as they're in proportion. I'd say just forget they're eyes, and draw them as basic shapes. Then step back and see if you got the expression you wanted. If not, at least you've got the proportions right, and you can work from there. And hey, everything else in them looks fine and dandy, and this is just my opinion, so take it w/ a pinch of salt...
I agree with Ninjacat that you should maybe put some more work on plain linework to try and make the figures stand out more and have a better base from which to build volume and stuff. But hey, I'm the line- only guy, so maybe that's me
Keep it up, dude, 'cause it sure is going somewhere nice!