Harry spends the weekend doing homework, but he feels satisfied knowing that he's rebelling against Umbridge. It's also heartening to know people believe he's not lying. On Monday morning, Harry and Ron join students around the bulletin board to inspect a new sign. It's an order from Umbridge, saying that all student groups are disbanded and must seek permission from Umbridge to re-form. Harry knows that this isn't a coincidence, and Ron suggests that Zacharias Smith told Umbridge about their meeting. He turns to run up the girls' staircase to tell Hermione, but the stairs turn into a slide—a way of keeping boys out of the girls’ dorms. Hermione slides down a few minutes later. Though she looks upset about the notice, she says that they'd know if someone told—she bewitched the parchment they signed, and if a person tells, they’ll break out in terrible acne.
Notice that Ron's first instinct is to blame someone who attended the meeting, not any of the unknown patrons at the Hog's Head. This again illustrates how Hogwarts is set up in such a way as to encourage this kind of suspicion, rather than show students that they're all on the same team. Hermione's curse on the membership list attaches major consequences to ratting out the group, which shows that what Hermione values most of all right now is loyalty—and rightly so, given what's at stake with Umbridge.
In the Great Hall, all the students seem on edge about the notice. Harry mutters to Fred and George that they're going to hold meetings anyway. Hermione motions for Hufflepuff and Ravenclaw members to not come over to chat. A bit later, Angelina tells Harry and Ron that the Gryffindor Quidditch team has to apply for permission to re-form, and she begs Harry to behave for Umbridge.
Holding the meetings anyway allows Harry to feel as though he's effectively rebelling against Umbridge. The fact that this rebellion is quiet also speaks to what Harry learned over the summer at Grimmauld Place: resistance doesn't need to be loud or flashy to be effective.
A few minutes into History of Magic, Hermione pokes Harry and points to the window. Hedwig is there. Harry slips out of his seat and lets Hedwig in, noticing that she's clearly hurt. Harry tells Professor Binns that he's ill and asks to leave class. He goes to the staff room to find Grubbly-Plank. McGonagall opens the door and Grubbly-Plank inspects Hedwig, reasoning that the Hogwarts thestrals wouldn't have gotten her. Harry says that Hedwig came from London, which makes McGonagall raise her eyebrows. Grubbly-Plank offers to take Hedwig for a day, and as the bell rings, McGonagall warns Harry that communication in and out of Hogwarts is being monitored.
All the signs and McGonagall's warning point to the possibility that Umbridge is targeting Harry specifically, rather than trying to repress the entire student body in all the same way. Grubbly-Plank’s casual mention of thestrals introduces the name of the creature that Harry will later connect to the invisible, spectral horses. The fact that Hedwig is hurt might mean that someone intercepted her on her journey, intending to read Harry’s mail.
Hedwig's letter is from Sirius, and says that he'll talk to them again in the fire. Harry shares everything with Ron and Hermione, and they wonder if someone tried to intercept Hedwig. Hermione worries that they can't even warn Sirius not to talk to them without that being intercepted. Down in the dungeons at Potions class, Malfoy boasts that Umbridge already gave the Slytherin Quidditch team permission to practice. He insults Mr. Weasley and says that Harry might still end up in St. Mungo's hospital, since he's insane. At this, Neville tries to charge Malfoy, but Harry and Ron pull him back. Harry doesn't tell Ron that Neville behaved like this because his parents are in St. Mungo's.
Neville, like Harry, is keeping secrets that in turn keep him isolated and unknowable in the eyes of his classmates. Though Neville is of course entitled to share or not share the truth about his parents, sharing this information could help Neville feel more integrated into the student body and make him a more sympathetic individual in the eyes of others.
Snape introduces Umbridge to the class—she’ll be inspecting—and Harry tries to decide which professor he wants to win this battle. In his desire to listen to Umbridge and Snape's conversation, he makes mistakes in his potion. Umbridge suggests that the class shouldn't be working on such advanced potions, and she asks Snape about his unsuccessful applications for the Defense Against the Dark Arts position. Snape angrily Vanishes Harry's foul-smelling potion and assigns him an extra essay. In Divination later, Trelawney seems oddly upset and explains to Parvati and Lavender that she's on probation.
To many, working on advanced material would be the mark of a good teacher—it would imply that the students already have a firm enough grasp of basic concepts to expand what they're doing beyond grade level. This again shows that Umbridge is trying to figure out how to use Hogwarts to control what and how much students learn—and an uneducated population is much easier to control.
After dinner, Angelina tells Harry and Ron that Umbridge supposedly needs time to think about whether Gryffindor can play Quidditch. Harry spends his evening trying to concentrate on his homework while checking the fire for Sirius and watching Fred demonstrate the effects of the Puking Pastilles. Harry can't figure out how Fred and George each only got three O.W.L.s, since they're clearly knowledgeable and making money.
Harry's comment about Fred and George's poor O.W.L. scores reminds the reader that there are more ways to be successful in life than just doing well on standardized tests. The fact that Fred and George are successfully selling their wares, and can develop them in the first place, shows that test scores aren't always a direct correlation to intelligence.
A bit later, when the common room is empty, Sirius's head appears in the fire. He grins and says he knows about the secret Defense Against the Dark Arts group, as the Hog's Head was the worst choice for a meeting place. He says that Mundungus was dressed as a witch and overheard them. Sirius says that Mrs. Weasley doesn't want Ron to participate, but she can't say so herself since she's "on duty" tonight. Sirius says that he believes the organization is a great idea, even if they do get expelled—at least they'll be prepared. He helps them brainstorm where to hold meetings, but then he disappears suddenly. Hermione gasps and she, Harry, and Ron race away from the fireplace—they can see Umbridge's hand grabbing about in the flames.
Sirius amplifies and reinforces Harry's suspicions that test scores aren't everything, and certainly aren't the most useful things to have as an adult. He says clearly that it's better to have the information than the scores, which, coming from an adult that Harry trusts and admires, means that Harry takes this to heart. Mrs. Weasley being "on duty" for something keys Harry into the fact that the Order is guarding something, and it requires overnight shifts to do so. Umbridge is clearly increasing her surveillance of the school, as she’s even searching the fires now.