The Unrequited, by H. B. Prince (гет)

Harriet is back at Hogwarts for a final year. She doesn't quite know why, but what matters is that she is here.
Back at Hogwarts too, is Severus Snape. Most students don't quite know why – and they certainly wish he weren't – but what matters is that he is here. In particular, Harriet finds it matters to her.
There are also new additions to the school this year: werewolves enrolled in the brand-new Lupin Programme.
(Also, Hogwarts is back at Hogwarts, and it does know why.)