Problems at Hogwarts
Ron and Hermione are named prefects, leaving Harry somewhat envious. At Hogwarts, they are surprised that Dolores Umbridge, Senior Undersecretary to the Minister and who presided at Harry's hearing, has been appointed as the new Defense Against the Dark Arts teacher. Hermione infers that the Ministry is interfering with Hogwarts, and it is soon apparent that Hermione is correct; Umbridge teaches only Ministry approved theory rather than practical defence methods, and appears to be there primarily to spy on the school. She is soon appointed as High Inquisitor, arbitrarily imposing strict rules and regulations. She also harbours racial hatred for "half-breeds," such as centaurs, werewolves, and similar creatures. She considers Rubeus Hagrid (a half-giant) and Sybill Trelawney incompetent. Trelawney is soon sacked, and Hagrid is dismissed later as well. Although Dumbledore is unable to prevent Trelawney's dismissal, he invokes his authority to allow her to remain in the castle and appoints a new Divination teacher ? the centaur, Firenze.
Harry has been having disturbing dreams about running down a hallway and attempting to open a door in the Ministry of Magic's Department of Mysteries. He also dreams he is a snake attacking Ron's father. Mr. Weasley is indeed found injured at the Ministry, suffering from severe venomous snake bites, which causes Harry to fear that he is being possessed by Voldemort. In response, Dumbledore has Severus Snape teach Harry Occlumency to block his mind from intrusion, but their mutual animosity ends their lessons prematurely.
Hermione blackmails journalist Rita Skeeter into writing a favourable article about Harry witnessing Voldemort's return. Ravenclaw student Luna Lovegood's father publishes the story in his magazine, The Quibbler. Furious, Umbridge bans the tabloid from the school. The story spreads rapidly, however, gathering support for Harry.
Dumbledore's Army and the Student Revolt
Hermione convinces Harry to secretly teach students Defense Against the Dark Arts. They name their clandestine group "Dumbledore's Army", or "D.A." for short, to mock the Ministry of Magic, which fears Dumbledore is creating a secret wizard army. Under Harry's tutoring, the group, consisting of Gryffindors, Ravenclaws, and Hufflepuffs, learn how to defend themselves against the Dark Arts. At the same time, Umbridge created the Inquisitorial Squad, a group mainly consisting of Slytherins, who had more power than prefects themselves. When Umbridge and her Squad uncover the D.A.'s meetings, Dumbledore claims that he organized it, to protect the students from reprisal. Confronted by two Aurors (Dawlish and Shacklebolt), Minister Fudge, Percy Weasley and Umbridge, Dumbledore easily overpowers them and is spectacularly whisked away by his phoenix, Fawkes.
Umbridge is appointed Headmistress and imposes even tighter control on the school. Fed up, The Weasley twins instigate and lead a revolt, creating non-stop chaos throughout the school. The staff purposely do nothing to help Umbridge regain control. Fred and George are caught, but summoning their confiscated brooms, they zoom off, leaving the school for good to open their own joke shop in Diagon Alley, Weasleys' Wizard Wheezes.