Harry doesn't know if Hermione even has a plan as he follows her out of the school, Umbridge on their heels. Umbridge asks if the weapon is in Hagrid's hut, and then reasons that because he's a "half-breed oaf," he'd set it off. Hermione leads them into the forest. Harry is concerned when she takes the path leading to Hagrid's giant spiders, especially when Hermione makes lots of noise. When they're a bit ahead of Umbridge, Hermione whispers that she wants them to be heard. A few minutes later, an arrow lands in a tree above Hermione's ahead. Centaurs surround them and point their bows at the humans. Umbridge looks terrified; Hermione looks triumphant.
Especially now that Umbridge feels as though she's powerful and in control, she feels even freer to use offensive language to make herself feel superior. The fact that Umbridge looks terrified of the angry centaurs confirms that what's really at the root of her hate of difference is fear. Hermione has thus attempted to weaponize Umbridge’s bigotry against her, as she knows the centaurs are dangerous and Umbridge is likely to anger them.
Magorian approaches and roughly asks Umbridge who she is. Umbridge squeaks that she's from the Ministry and reminds Magorian that as a half-breed, he could get in trouble for attacking her. Bane mutters angrily as Umbridge goes on about centaurs' "near-human intelligence." She says that the Ministry owns the forest, insults the centaurs, and conjures ropes to bind Magorian. The centaurs charge at Umbridge as Harry pulls Hermione to the ground. Bane lifts Umbridge into the air and carries her away.
Because Umbridge doesn't believe that the centaurs have any power and should be afraid of her and other humans, it never occurs to her to actually fear what they could do to her if they wanted. Their reaction suggests that their anger has been simmering for a long time, and this reaction has been a long time coming given the way that the Ministry treats centaurs.
Another centaur lifts up Hermione and Harry. The group discusses whether Hermione and Harry are young enough to spare, and Hermione says she hoped that they'd drive Umbridge off for them. The centaur shouts that Hermione is already arrogant like other humans. Hermione begs to be allowed to return to school. Then they hear a crash. Grawp peers around some trees, notices Hermione, and asks where "Hagger" is. Hermione realizes he's looking for Hagrid and tries to tell him she doesn't know. As Grawp reaches for Hermione and hits a centaur, the centaurs shoot arrows at his face. Harry and Hermione run as Grawp and the centaurs shout and fight.
This centaur is correct in that Hermione is abusing the centaurs by orchestrating a situation in which they do her dirty work—which, given the way that Umbridge talks about the law, could ultimately get the centaurs in a great deal of trouble with the Ministry. This reminds the reader that even Hermione still has a long way to go as she learns about the diverse inhabitants of her world and learns how to appropriately interact with them and take their concerns seriously.
Once they're far enough away, Harry kicks a tree, thinking that they've wasted too much time. Ron, Neville, Ginny, and Luna appear with Harry and Hermione's wands and ask how they're getting to London. They explain that they broke free from the Slytherins. Luna mysteriously says they can fly, while Ron and Harry try to tell the others that they can't come. Neville insists that they were in the D.A. together, so they should go together. They return to the question of how to get there, and Luna points behind Harry to a few thestrals. More arrive, clearly drawn by the smell of Grawp's blood on Harry and Hermione. Harry relents and tells everyone to get on.
Neville's insistence that everyone should go to the Department of Mysteries shows that he's learned over the year that he can and should rely on his friends to help him when he needs it, just as he now assures Harry that he's here to help him. When Luna points out that they can take the thestrals to London, Harry is reminded that he shouldn't underestimate those around him, as everyone has something to offer.