Oh, this is fabulous. There needs to be more Erised-fic -- I've always thought the Mirror was one of JKR's more fascinating inventions -- and Phineas the jaded academic is absolutely delicious, especially when you see his human side break through. I particularly like this bit:

Phineas, for a moment, saw Hogwarts transformed into a place of higher learning to rival Wittenberg, and himself at the head of a body of adult scholars, while his eldest son, forsaking his disdain for the academic, presided over the school. “Shut it up” he said wearily, and Phillip did so.