Lavender asks Matilda how the Trunchbull can get away with this; her father would be very upset if the Trunchbull swung her by the hair. Matilda says that Lavender’s father wouldn’t believe her. The Trunchbull knows that she has to be outrageous to get away with it. No parent will believe a headmistress swung a student by her braids. When Lavender notes that Amanda’s mother will cut Amanda’s braids off, Matilda says that Amanda will cut her own braids. Then, she says the Trunchbull probably isn’t mad, but she is very dangerous.
Matilda suggests here that even caring parents who want to protect their kids sometimes fail to do so. Sensible adults, Matilda realizes, aren’t going to believe Miss Trunchbull is as bad as she actually is, which leaves their kids vulnerable to abuse by Miss Trunchbull. This puts kids like Amanda in a position where they have to sneak around and, in this case, cut her own braids off to keep herself safe.
The next day during lunch, the students are told to go to the Assembly Hall after the meal. When everyone is seated, the Trunchbull takes the stage with a riding crop and calls a boy named Bruce Bogtrotter to the stage. Bruce is a round boy and very nervous; he eyes the Trunchbull’s crop warily. The Trunchbull announces to the students that Bruce is a thief. Bruce has no idea what she’s talking about, but the Trunchbull accuses Bruce of stealing her piece of special chocolate cake from her tea tray yesterday. Unlike food for the students, her cake was made with real butter and cream. Bruce denies the accusation.
Recall that Miss Trunchbull is upset in general because she isn’t allowed to hit kids anymore—so the riding crop is just a prop designed to terrify poor Bruce. So Miss Trunchbull doesn’t have to even touch the students to gain power over them. It also seems like Miss Trunchbull continues to abuse her position as headmistress if she’s getting food made out of real ingredients while the children eat something else presumably not as good.
Then, the Trunchbull leans down to Bruce and quietly asks if he liked the cake. Bruce can’t help himself—he says the cake was very good. The Trunchbull agrees and says that, to be polite, Bruce must thank the chef. She yells for the cook to enter the hall and coaches Bruce through telling the old lady that the cake was wonderful. Bruce knows the Trunchbull can’t hit him with her riding crop, but he’s certain something bad is going to happen.
Miss Trunchbull isn’t, as a rule, polite—she throws kids by their braids and threatens them with riding crops. So this display of supposedly coaching Bruce on how to be polite reads as extremely concerning and dangerous for Bruce. Though Bruce can’t rely on any other adults to help him, it is some solace that the law preventing corporal punishment is at least on his side.
The Trunchbull asks the cook if she has more cake to share with Bruce. Right on cue, the cook disappears and returns with a huge cake, 18 inches in diameter, and a knife. Once the cake is on the table, the Trunchbull asks Bruce to sit down and invites him to have a slice of this cake, which is all for him. Bruce thanks the cook, but he is hesitant to eat now. The Trunchbull forces him to cut a slice. All the students wonder if there’s castor oil or pepper in the cake, or maybe poison. Bruce dutifully eats a tiny slice—but the Trunchbull tells him to have another. The boy doesn’t want one, but the Trunchbull says Bruce got exactly what he wanted: cake. He’s not leaving until the cake is gone.
It's impossible to tell if the cook is going along with Miss Trunchbull because she agrees with the headmistress, or because, like the students, she’s terrified of her boss. So Miss Trunchbull might have the entire staff at school cowed. When it comes to forcing Bruce to eat the entire cake in front of all his classmates, the goal here is clearly to humiliate Bruce and show the other kids what happens when they cross Miss Trunchbull. This shows that Miss Trunchbull excels at coming up with punishments like this, that terrify children without having to actually hit them.
Matilda and Lavender watch, fascinated, as Bruce eats three slices of cake. They’re not sure he can do it—especially with the Trunchbull calling him greedy and threatening to put him in the Chokey. When he’s halfway through the cake, Bruce starts to eat faster. The watching students are now rooting for Bruce, who seems like he’s going to succeed. He almost seems like he’s enjoying it, especially when someone shouts out encouragement to Bruce. Lavender and Matilda are shocked and watch as Bruce shoves the last bites of cake in his mouth—and the Trunchbull turns red with anger. When he’s done, the students all cheer. The Trunchbull picks up the platter and breaks it over Bruce’s head. Bruce, though, is so full of cake that it doesn’t hurt him.
Before Bruce started to do well, it was a bit of a liability for the other kids to root for him. It’s dangerous, after all, to openly oppose Miss Trunchbull and stand up to her abuse. But when Bruce manages to beat Miss Trunchbull at her own game by finishing the cake, it’s much safer for the other students to celebrate—they’re several hundred against one, after all. When Miss Trunchbull still ends this torture session by breaking the plate over Bruce’s head, it shows that whatever the law says, she still consistently turns to physical violence to get her way and make a point.