Chapter 31 of Harry Potter and the Order of the Phoenix: O.W.L.s
Ron's elation over his performance and Gryffindor's victory lasts well into the next day. Harry and Hermione let him enjoy it before telling him they missed the match because Hagrid dragged them away to see Grawp. Ron is incredulous that Hagrid brought back a Giant and reluctant to help care for one, much less teach it English. "He's lost his mind!" Hermione concurs, but says they promised to help Hagrid.
The Fifth Year Ordinary Wizarding Level examinations begin, and Harry is relieved that many questions cover familiar information. In the first week, he performs creditably in Charms, Transfiguration, Herbology, and Defence Against the Dark Arts. Harry receives an extra credit for producing a Patronus when requested. Professor Umbridge, who is observing, seems pleased, but Harry hardly cares. After her Ancient Runes exam, Hermione reports that someone put another Niffler into Umbridge's office; it will be another excuse to sack Hagrid.
The following Monday, Harry does reasonably well in the Potions exam as Snape is absent. Tuesday is Care of Magical Creatures, and Wednesday morning is Astronomy. Both Harry and Ron fail Divination miserably on Wednesday. During the evening Astronomy practical exam, Harry spies Umbridge and a group heading for Hagrid's hut. A battle breaks out and Stunning spells are cast at Hagrid, though his massive body deflects them. Professor McGonagall runs to Hagrid's aid, but Stunning spells knock her down. Hagrid escapes, and McGonagall is rushed to the hospital wing.
Later in the common room, Hermione comments that the Nifflers in Umbridge's office will be used as a pretext for sacking Hagrid. Lee Jordan admits that he put them there, but Hermione claims that Umbridge, hating what she calls "half-breeds", would have sacked him anyway.
One thing that is mentioned in the text, but never explained, is that Umbridge seems pleased that Harry, the target of her ire from well before her first day of teaching, is able to produce a Patronus. (The exact words in the book are There was a nasty smile playing about her mouth.) We can only speculate at this, as Harry does not choose to examine the incident further. It is possible that Umbridge is pleased because it will give her a second chance (she believes) to attack Harry in the following year, or that it reflects well on her "skills" as an instructor. This actually is the one drawback, had Harry thought of it at the time, of Dumbledore's Army: by providing competent teaching of Defence Against the Dark Arts, Harry at least partly manages to mask Umbridge's incompetence. One must wonder how any students from Slytherin house in Harry's year made sufficiently high OWL grades to study advanced Defence Against the Dark Arts, being taught only by Umbridge in their critical fifth year.
Ron's opinion regarding how wise it is to attempt to civilize Grawp clearly echoes Harry and Hermione's sentiment. Because Hagrid is absent, Ron alone is able to verbalize the entire Trio's doubts, and does so with his usual outspokenness. And unlike Harry and Hermione, who personally witnessed the lonely Hagrid's nearly impossible attempt to civilize his only relative, Ron's opinion is far less affected by his emotions. The author points up Hermione's unhappiness over them having promised to assume this task, in the event that Hagrid gets sacked. Hagrid is sacked, most dramatically, and the Trio is now responsible for visiting Grawp occasionally, despite the Forbidden Forest being off-limits to them, and against the Centaurs' wishes. Hagrid's request puts the students in danger, but his need to protect his half-brother has perhaps blinded him to just how perilous his request is.
Harry, meanwhile, soon has more pressing problems as he experiences a vision that Voldemort is torturing Sirius at the Ministry of Magic.
- What is Ron's reaction when Harry and Hermione tell him about Grawp?
- What does Harry witness during his Astronomy exam? What happens to McGonagall?
- What does Harry "see" during his History of Magic O.W.L. exam?
Further Study 
- Umbridge seems pleased when Harry conjures a Patronus in his Defence Against the Dark Arts OWL. Why? This is not something that she has taught; why does she not wonder where he had learned it?
- Sirius gave Harry a package that Harry could use to reach him, Sirius had said so. Why has Harry forgotten this?
- Is the dream a hoax, a trap, or is it real?
- There are several pieces of evidence that might lead Harry to doubt the veracity of this latest "dream" or "vision". Why does Harry not think these through?
- How could Voldemort and Sirius have managed to enter the Ministry unnoticed?
Greater Picture 
Following his dream, Harry will be in a panic over Sirius, positive he is in danger, but it seems there is no one he can to turn to. Umbridge is in complete control, Dumbledore and Hagrid are now gone, Harry will shortly find that Professor McGonagall has gone to St. Mungo's, and there seems little that Ron and Hermione can do. However, Harry is overlooking the one person left who can help him: Professor Snape, also an Order member. He has also forgotten (deliberately) the package Sirius gave him at Christmas, which Sirius had said would be a way to reach him at any time.
We will shortly find that Harry's dream has been created by Voldemort, as a means of luring Harry into the Department of Mysteries at the Ministry. Voldemort has been seeking an object, a Prophecy, which is contained in one of the small glass orbs that Harry sees in his vision. These orbs are charmed such that only those to whom they refer can safely touch them; Voldemort knows that only he and Harry can retrieve the prophecy from the shelf where it lies, and has determined to trick Harry into doing it for him.
During the chaotic events, the Trio will quite forget their promise to visit Grawp. However, the time involved will be very short: the Astronomy exam, we see here, is Wednesday night, which is when Hagrid is sacked; the History of Magic exam, Friday afternoon, is when Harry has his dream, and they will visit the Ministry this same evening. That visit will culminate with a battle, which will result in Dumbledore's reappearance and the Ministry admitting that Voldemort has returned. One of Dumbledore's demands is that the Ministry stop chasing his groundskeeper, meaning that Hagrid is free to return to Hogwarts only two days after having been driven away.