Outside [Potions] [Malfoy] rattles off some of his usual insults, but when he makes a crack about the long-term ward at [St. Mungo's], [Neville] tries to attack him. [Harry] and [Ron] hold him back, and [Snape] promptly takes [points] off all of them for fighting.
A Letter From Sirius
[Harry] opens [Sirius]'s letter, which says he'll be back in the [common room] fire this evening, and shares it with a worried [Ron] and [Hermione].
[Umbridge] is once again at the back of [Harry]'s class, this time inspecting [Snape] in [Potions]. Snape does alright with the inspection, but gives Harry, who has listened to the entire thing, zero marks once again.
Professor Trelawney is on Probation
[Harry] briefly considers skipping [Divination], but [Hermione] talks him out of it.
In [Divination], [Harry] and [Ron] find a furious [Professor Trelawney] and soon realize that she has been placed on probation by [Umbridge].
As [Defence Against the Dark Arts] begins, [Harry] tells [Hermione] about [Trelawney] being placed on probation.
[Angelina] tells [Harry] and [Ron] that [Umbridge] has not yet given permission for the [Gryffindor Quidditch team] to re-form; he instead sits down to do his homework for [Snape] while watching [Fred], [George], and [Lee Jordan] demonstrate [Puking Pastilles].
[Sirius] appears once again in the fire of the [common room], and talks with [Harry], [Ron], and [Hermione] about their secret defence group. Suddenly, though, he disappears, and [Umbridge]'s hand appears where his head once was - she having just missed capturing him through the [Floo Network].
Chapter 18 - "Dumbledore's Army" - In which Harry again dreams of the windowless corridor with the door at the end, Dobby tells Harry about the Room of Requirement, and Dumbledore's Army meets for the first time.
[Hermione] spends [Charms] class telling [Harry] and [Ron] her theory that [Umbridge] has been reading all of Harry's correspondence, including his near-miss the previous night in which [Sirius] was nearly caught.