Oh, nicely done!
I loved Harry's suggestions as to who might be helping him (as everyone else has already said;-), and I'm glad to see that some of the Slytherins are getting sense. Snape's attitude is refreshing--he seems too much a realist not to see how the powers are shifting and to try to get himself an in with the new force on the scene.
Pansy and Marietta are idiots, but I suppose that's not a great surprise. The creative punishment was a nice touch, especially with it being reversable, or mostly so.
Harry and Hermione's discussion about Dumbledore was nice. I like Harry's obliviation of Dumbles, and am looking forward to seeing his plans for further declawing the MOB.