Matilda has an easy time making friends with the other children in her class, since she is very humble and polite and shows no outward signs of her brilliance. She becomes especially close friends with a girl named Lavender, and each likes the other because she is gutsy and adventurous. Many of the older kids warn them about Miss Trunchbull, and a ten-year-old named Hortensia informs them that "the Trunchbull" hates the youngest class the most.
She also tells them about The Chokey, a tiny cupboard in Miss Trunchbull's quarters where children are punished by being forced to stand up straight for hours, since the door and walls are made of glass and nails. Hortensia has been locked in it six times for various pranks she has pulled on Miss Trunchbull. Lavender and Matilda are in awe of Hortensia's mastery of messing with the Trunchbull.
Hortensia recounts a number of terrible things Miss Trunchbull has done to the children, including throwing one child out the window for eating in class. She tells them that the headmistress once threw hammer in the Olympics for Britain, so her arm strength is unparalleled. As she is explaining this, the playground falls silent and Miss Trunchbull stomps outside, parting the sea of children and shouting for one called Amanda Thripp. Amanda's mother has braided her long hair into pigtails, and Miss Trunchbull hates pigtails more than anything else.
Miss Trunchbull tells Amanda to chop off the pigtails and get rid of them before she comes to school tomorrow, but Amanda protests, saying her mother does them every morning and thinks they look pretty. Angry, Miss Trunchbull grabs the little girl by her pigtails and swings her around and around, then throws her the way she would throw a hammer in the Olympics. Amanda flies over the fence and outside the playground but, miraculously, she hops up once she lands and totters back.
When Matilda asks why children's parents do not complain about Miss Trunchbull, Hortensia tells her that the parents are just as afraid of her as the students. Lavender says her parents would raise a stink if she told them about her, but Matilda doubts that any parent would believe a story that sounds so ridiculous. Matilda says that's Miss Trunchbull's mindset: do things that are so outrageous that they cannot be believed, allowing her to get away with it.
Miss Trunchbull's antics continue the next day, when all two hundred fifty students are called to an assembly during lunch. Miss Trunchbull calls up a boy named Bruce Bogtrotter, accusing him of stealing a slice of her special chocolate cake from her tea tray. Bruce denies it, but she refuses to believe him. As punishment Miss Trunchbull calls out the school cook, who brings an entire enormous cake and tells him to eat a slice, right there. The students worry that she has poisoned the cake in some way, but she has not; instead, she intends to make him eat the entire cake on his own in front of everyone, and no one can leave until it is finished. It is a grueling process, but he does it, polishing off the entire plate in a triumphant victory. Miss Trunchbull is furious that he was able to.
Not long later, Miss Honey announces to Matilda's class that Miss Trunchbull has a policy of taking over each class for one period each week. On Thursday afternoons, it is their class's turn. She gives them instructions to be very careful about their appearance and behavior, since the headmistress is quite strict. She assigns Lavender to the task of preparing a jug of water to await the headmistress on her desk when she comes in. Lavender has a brilliant idea, and catches a newt from her garden to slip inside Miss Trunchbull's water jug the next day.
Dahl characterizes Matilda's time at school with an extended metaphor of battlefields and soldiers. Hortensia describes Miss Trunchbull's antics with an air of mystique and trepidation befitting an enemy army, and Matilda resolutely takes on the responsibility of banding with her fellow classmates and fighting back against the injustices of their headmistress. Hortensia gives a speech that exemplifies this metaphor of war: "We are the crusaders, the gallant army fighting for our lives with hardly any weapons at all" (Chapter 10, pg. 89). While it is often difficult, this battle unites the students, bringing them together in a way that Matilda has never experienced with children her age.
But this battle is about more than just the students vs. Miss Trunchbull: it is a battle of youth vs. age, of kids vs. adults. Miss Trunchbull is an "adult" in the negative sense of the word children ascribe to it: she is rude and cruel and abuses the power she has over the children, forgetting what it once felt like to be in their shoes. Like Matilda's parents, she is a physical manifestation of Dahl's message that adults are not always wiser or nobler than children. Matilda and her classmates, in turn, are representative of all children who feel oppressed by the adults in their lives. Thus, this war brings the forces of youth and age into conflict.
While the war is certainly fraught with struggle, there are small victories as time goes on, in which the children are able to triumph over Miss Trunchbull and even beat her at her own game. The incident with Bruce Bogtrotter is a prime example. Miss Trunchbull sought to make a fool out of Bruce as punishment, expecting him to be unable to finish the entire cake or to get sick while doing so. Instead, Bruce does exactly as she asks and eats the whole cake, and is able to walk away victorious. These little victories parallel Matilda's pranks against her father, and they give all the children hope that even though they are small and subordinate, they can still use their skills and cleverness to outsmart cruel adults.
Characterizing Matilda's school life as a war also keeps the stakes high, granting legitimacy to hers and her classmate's problems by blowing them up to extraordinary proportions. This is evident in the language Dahl uses in these chapters, particularly in Chapter 12, when he compares Lavender's plot to put a newt in Miss Trunchbull's water to the Duke of Wellington at the Battle of Waterloo (Chapter 12, pg. 110). By likening Miss Trunchbull to the enemy French dictator Napoleon, Dahl makes the children's struggle seem larger and more serious, raising the stakes.