When school bully Lavinia accuses Sara of making up "fairy stories" about Heaven, Sara primly tells her that no one knows what Heaven is really like, but unless Lavinia starts being kinder to people, she'll never find out - a fairly classy schoolgirl way of telling Lavinia to go to hell.
Ermengarde's announcement: "There WERE diamond mines. THERE WERE!"
From the 1995 film adaptation:
The scene in which Sara witnesses Miss Minchin abusing a little boy who works as her chimney sweep. In order to get back at Minchin, Sara and Becky climb up to the school roof and dump copious amounts of soot through the parlor fireplace flue while Minchin practices the harp. The result is a veritable mushroom cloud of soot and a wailing Minchin.
Midway through the film, Sara's friends engineer a plan to get back the locket Miss Minchin took from her. This involves a Cacophony Cover Up (Lottie's tantrum), sneaking into the headmistress' office, and going through her desk and shelves to find the locket. What makes the whole scene more awesome is that the girls are mere seconds from being caught, until Becky, who is outside the office, screams and tells Minchin she saw a mouse.
Sara's speech to Minchin: "I am a princess. All girls are. Even if they live in tiny old attics. Even if they dress in rags, even if they aren't pretty, or smart, or young. They're still princesses. All of us."
Sara's method of getting her revenge on Lavinia - a proper Mind Screw by having her believe that she is cursed.