Brilliant story -- one of my favorites of the fest so far. You've nailed adolescent Snape's voice exactly: intelligent, highly verbal, sardonic, callow, cynical, yearning. And Peter is a gem, too -- what a great job you've done of extrapolating subtle canon hints (Peter is too crafty and successful to be genuinely inept) that he is one of those capable people who, for sad, bad, and understandable reasons, chooses to hide his light under a bushel.
The Marauders, the believable backstory for "The Prank," the poignance and pragmatism of Peter's story of his mother, your thematic use of Transfiguration, the dialogue that technically says so little yet reveals so much, the frighteningly plausible attraction of Voldemort's cause to such spurned young men: all these elements are so well done and add up to a wonderful work. It affects me like a piece of jazz, a winding and melancholy riff on "nothing" and on those people whom the world would define as "nobody."
Here's just one example of your exquisite touch with style and mood: Then he spots it scuttling away over the grass – a squirrel or a rat. Some small creature. Nothing.. So few details, so spare, and yet so telling. "Nothing," indeed. Peter in microcosm in less than 20 words.