I thought that this was a great fic.
Beautiful. You have cast a bit of light on a side of Snape I never really think about, the bitter but reassuring routine he has found in Hogwarts to cope with his sorrow. It's good the way he uses what happens around him for his class plans, sounds very in character. The title of the chapter is also perfect, it conveys the ritual and at the same time hints that this year said ritual will be shattered.
I know you wrote this for a Challenge, but as with your other portrayals of Severus Snape, it does seem so very much in character. The ambivalence over teaching the course, the realisation of what it means when he receives the letter, the tone in which you write his thoughts-- all are so very much in keeping with his character. List of favorites: your observations about the two worst things happening, and his Boggart, and the part where he corrects himself and comments that rather than being "wrong beyond repair" something must just be "beyond (my) immediate influence."