Thank you, thank you!! This is wonderful. In particular, I love the way you let us see the staff as real people, puzzling this new guy out. And it's great that you kept Snape recognizeable as his canon character, yet show him through adult eyes and suddenly we see his good qualities. It works well to have vignettes through the years, and I like the way you show each house head taking their own approach with him. I enjoyed the byplay with Flitwick (and yes, I can believe Snape would have real respect for him) and McGonagall is marvelous here, especially with her stubborness after he's gone and her reaction to seeing him again.
I'd end up quoting large chunks of the story to you if I were to point out lines that really hit home. But there were two in particular. The first, "It typically took professors at least an entire term to get their footing and figure out what method they liked best. It took Severus Snape three days" really made me laugh out loud. And my heart twisted just a bit when Minerva realized that he was hurt by how happily his students took to Slughorn, even when he wouldn't admit it.