At the end of the last chapter, Hermione launched an impromptu plan to free herself and Harry from Umbridge, who had caught them using her office fireplace to talk to someone. Hermione’s plan was to pretend that they were talking to Dumbledore, telling him that the powerful secret weapon he’d created was ready to use. Hermione pretended to break down and say that she was willing to lead Umbridge to it. Umbridge fell for the ruse and forced her and Harry to lead her to the weapon, which Hermione told her was secreted away in the Forbidden Forest. 
In this chapter, the plan works. Well, sort of works. Okay, the only reason it works is because of the fortuitous arrival of Grawp. If the giant hadn’t shown up, things would have worked out very badly indeed for Harry and Hermione. 
What went wrong? More to the point, what exactly was Hermione’s plan? What was she thinking?
From the things she says to the Centaurs, it’s clear that her plan was to bring Umbridge into the Forest and let the Centaurs attack her. But why would she expect them to do that? Hermione has had very limited experience with Centaurs. She first encountered them in her first year when she, Harry, Neville, and Draco were brought into the Forest in the middle of the night by Hagrid as a detention. In that encounter, the Centaurs seemed a bit aggressive but not hostile. The fact that Hagrid was present, however, made that encounter far from typical. Hermione would certainly have understood the unusual nature of that meeting.
Hermione’s second encounter with Centaurs would be second hand at best. Since she gave up taking Divination, she wouldn’t have had Firenze for a teacher. However, she would certainly have heard the other girls in her dorm talking about him and how he was rejected by his herd for fraternizing with humans. Would this have led her to believe that the Centaurs in the Forest would be hostile to witches and wizards?
Perhaps so, but she wouldn’t have had any reason to expect the Centaurs to treat her and Harry with any less hostility. In fact, as far as she knew she was leading all three of them into a very dangerous situation, not just Umbridge. And neither Hermione nor Harry was carrying a wand. 
It would have made a lot more sense for her or Harry to have overpowered Umbridge once they entered the Forest. At one point, Umbridge trips and falls over a sapling, a perfect opportunity to pretend to help her up but instead grab her wand. It hardly seems that Umbridge would have been much of a match for two healthy, determined teenagers. Alternatively, Harry and Hermione could have simply run away, splitting up and dodging between trees. Umbridge might have managed to Stun one of them — possibly. I don’t imagine that she’s all that powerful of a witch, a fact strongly suggested by the shortness of her wand, which is revealed in this passage from chapter 12:
Many of the class exchanged gloomy looks; the order ‘wands away’ had never yet been followed by a lesson they had found interesting. Harry shoved his wand back inside his bag and pulled out quill, ink and parchment. Professor Umbridge opened her handbag, extracted her own wand, which was an unusually short one, and tapped the blackboard sharply with it; words appeared on the board at once:
Defence Against the Dark Arts A Return to Basic Principles (OP12)
According to Mr Ollivander, a short wand does indicate something lacking in the character or ability of the witch or wizard possessing it. He writes:
However, abnormally short wands usually select those in whose character something is lacking, rather than because they are physically undersized (many...