The stresses of fifth-year finally get to Harry and he snaps, partially awakening the Horcrux in his head and it begins to influence him. It begins to fill his mind with advanced magical knowledge and dark thoughts. Some he begins to act on, not realizing the effect at first.
After a few weeks of this, and just after Dumbledore's departure from the school, Umbridge makes the mistake of prodding him in the Great Hall upon which he pulls his wand and tortures her in front of the students and professors, who find themselves powerless to stop his impressive display of magic.
Taunting her asking if she is happy now. If she is happy she broke him and now he can feel Voldemort's influence effecting him that there are two monsters walking around. The Inquisitorial Squad attempt to intervene and Harry wipes the floor with them, barely even trying and continuing to torment Umbridge in between strikes.
The teachers try to keep him from harming the other students, but after a one-on-one duel between Harry and Flitwick becomes a one-sided thrashing of the diminutive man, they realize they are thoroughly outmatched and each and every one of them assess just how badly they have failed Harry for this to happen. Thankfully for them, Harry takes it as a lighthearted fun duel, but warns them against any such foolishness in the future.
Hermione initially tries to help bring back the old Harry, but soon finds herself falling for this new confident powerful Harry and she is soon caught by his spell and begins to descend with him. Though through her efforts Harry overcomes and absorbs the power and knowledge of the Horcrux, he does not return to his old self.
They keep Umbridge and her Inquisitorial Squad locked in the dungeons and torture them for information, and to keep the turn of events from getting out, while having the DA teach the students to fight.
By the time news does leak to the Ministry and Voldemort, Harry has managed to turn most of the school into his own personal army, some with the same level of devotion as Bellatrix. And Harry is beyond done with the adults and their childish, halfassed version of war.