Hogwarts: I graduated before the system came

"Dumbledore, hasn't he graduated? Why is he in school again?"
"Oh, don't be nervous Severus, he's the new Defense Against the Dark Arts professor!"
"Oh, damn, if I see him eating hot pot from a cauldron at school again, I'll challenge him to a duel." ······
"You are the worst class I have ever taught!" "But, Professor, isn't this your first time teaching this class?" ······
"Miss Granger, you are the study committee member. You must lead them in morning reading by the Black Lake."
"Mr. Pot, you are the sports committee member. After the morning reading every day, take them for a run around the Black Lake!" ······
"One hundred days left until the OWL exam!" "Desperately strive for a perfect OWL!"
Douglas Holmes, a Hufflepuff with an oriental soul, returned to Hogwarts three years after graduation to apply for a professorship, bringing a new dimension to the school.