Perfection (or close, at least)
It's really a great game - the graphics were brilliant and fitted the game perfectly. I gave up on the final exam, though (lazyness). Perhaps you should make some sort of hints for the people who aren't as brilliant as you? It's quite a challenging game - and should stay that way.
But all in all a great game, basicly nothing to be improved, it was smooth and looked well. Adding a Help page for the grey boxes might be good as well.
Good job! It definently made me want to play more.