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
[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
[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.