Here's some advice from somebody that isn't in the industry. Take it for what it's worth. I'm no expert but I've looked at a hell of a lot of portfolios so here goes.
Your work is great. I especially like the creature designs. (That is the stuff that I always look for.) I don't think you should go back to school. Build on your strengths and improve your weaknesses through practice. I've heard it said that you're only as strong as the weakest piece in your portfolio. The idea is to get everything up to the calibur of your creature design.
I think the weakest part of your portfolio is the figure drawing. If you want to draw concepts for a living that will have to be improved. Their are plenty of figure books from the old masters to start with. Sneak into some figure classes at a local college. You'll have to forgive my honesty, but you'll never improve if I just stroke your ego. Believe me, I'm no Andrew Jones myself. Like I said before, I haven't even broken into the industry yet.
If you are more interested in the 3D modeling part of things then from what I've seen you don't have to focus as much on the figure drawing. (It's still important though and it almost always gets looked at.)
Hope I helped,