Oh, wow, Peter, such an amazing story. At the risk of overthinking it: I've always thought a good Marauders fic needs a Theory of Peter that goes beyond what's presented in the books, a way of explaining what canon doesn't explain, why things eventually went so catastrophically wrong among friends and allies. This story has that in spades. Peter's so clearly presented as someone knows about and values anonymity, flexibility, and self-preservation--and it's so beautifully evidenced in his approach to magic. Severus as a young man is still all about force and raw anger (no wonder vanishing spells are so hard for him), and later he'll be about control and precision, but Peter's about blending in, accommodating, waiting for his chance.
The details are what make this fic: the little signs of character (that fuck you he wouldn’t dare offer out loud, Peter and his "hiding emergencies," Severus threatening Peter with an Evanesco, not quite sure what he's doing, preoccupied with "showing them") as well as all the painful details of adolescent social life (friendships, alliances, inter-house competition, in groups, out groups). Gah! This fic brought back all kinds of bad memories about teenagers and how acutely aware we all were of our place in various social groups. The comment about enforced collegiality in the dining hall over the summer, Sirius's quiet friendship/relationship with Flint, and Peter's matter-of-face acceptance of his cohort ("friends") all rang so true to me.