There is a "dungeon" in Flitwick's Wingardium Leviosa room, under a trap door with a big shiny lock (You need Reducto to open it, which you don't get until year 4). When you jump in, you can get some loot (bring a Dark Magic user) -- but then you are seemingly trapped. THERE IS HOPE! Using Hermione, make your active spell Crookshanks. 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 few tries, and you will probably kill Hermione a bunch of times, but it will work.