Hogwarts: I have graduated before the system came

Introduction: "Dumbledore, didn't he graduate, why did he show up at the school again!" "Oh, don't worry, Severus, he's the new Defense Against the Dark Arts professor!" "Oh, damn it, if I see him eating hot pot with a cauldron at school again, I'll fight him." "You guys are the worst class I've ever taught!" "But, professor, isn't this your first class?" "Miss Granger, you're a study committee member , lead them to do their morning reading by the Black Lake every morning." "Mr. Pot, you are the sports committee member, take them to run around the Black Lake after the morning reading every day!"...