There is a "dungeon" in Flitwick's Wingardium Leviosa room, under a trap door with a largeshiny lock (You need Reducto to open it, which you don't access until year 4). When you jump in, you can get some loot (bring a Dark Magic user) -- but then you are seemingly trapped. Using Hermione, make your active spell Crookshanks. Then, go to the left top corner and jump out of the window while tapping the spell button. When it works, Crookshanks will appear in the classroom and you can walk out of the room. It takes a couple tries and you will probably kill Hermione several times, but it will work.