Lovely writing here! I liked this line especially:
Barely more than a boy himself, he stands out in the crowd of students, his black-clad figure gliding through their midst like a silent messenger of death, and Poppy shivers.
You do a great job of showing how alienated from Snape Poppy feels, both in the flashbacks and the present timeline, while also giving us hope that this might change. Also, I really felt how disconcerting it was for Poppy to be caring for a patient who was neither dead nor alive.