I love the expression that he has. The fade out of the background characters is fantastic. I dont think the purple on the face it right though. Maybe a lighter shade? I also think the water at the bottom looks to much like blood. You could try and make it more clear.
But i freaking love this piece. its exactly how i imagined him.
I snarfled when I saw Dennis's face! Great job the way you've led the eye straight to Dennis by keeping his lines sharper than the surrounding students'. It keeps the focus where it should be since you conquered the urge put detail where it didn't need to go. And LOVE the thumbs up! It makes an already winning expression even better!
On the puddle, it looks more like oil than water and I think it would have to be extremely dark in the Hall for it to be so black--try using more transparency so the paving stones show through, and some glint of the overhead candles too. I know they aren't pictured, but since the Great Hall uses them as their primary lighting source, a few twinkles on the puddle would add still more awesome (yeah me good with vocabulary wurdz).
You're right on the puddle, a little transparency wouldn't have hurt. I think I remember planning to work on it more before I submitted it, but I got lazy. Ah well, and thanks for the crit, its much appreciated!