Severus had long ago concluded the hormones levels of the older students in the spring could power the entire school with the magical energy they produced.
That line just cracks me up. Your opening is fabulous and totally had me hooked. ;) And he hexed the mermaid -- hahaha!
The references to Dumbledore -- and his sexual preferences -- are a nice touch here, too. Lots of fabulous little details in this. The wanking, of course, is most tasty, and the twist ending is great. Love that last line! Really well done. :D