This is Dumbledore's Army. It is a secret Defense Against the Dark Arts which was Hermione's idea. There Harry teaches everyone in the class real Defense Against the Dark Arts and everything they need to know about defending them self against Voldemort and other Dark Wizards. They have classes in the Room of Requirement, which only appears when you thing hard about something you really need. It is the best Defense Against the Dark Arts there is. They all helped Harry, Ron, and Hermione find Ravenclaw's Diadem.