Nicely plotted, but oddly undramatic.  I think you have an epic story here that's begging for more detail, drama, and suspense.  Of course, that'll mean tons more writing, so I sympathize!  I would suggest maybe fine-tuning some of the dialogue to suit the characters better.  I don't know if you're trying to sound like Rowling, but I dont' think Lupin would ever say something like "Yeah, all right."  Snape is a little more in character.