Despite the Trunchbull's comment that small girls are more evil than small boys—"'a bad girl is a far more dangerous creature than a bad boy. What's more, they're much harder to squash'" (8.11)—it seems like the Trunchbull picks on male students more than she picks on the girls, overall. Nigel, Rupert, Eric, and Wilfred each have the bad luck to come to her attention.
First up? Nigel, the smart and brave little boy who's the one to spell "cat" on the first day. Nigel gets in trouble with the Trunchbull because he's got dirty hands and a dirty shirt. He doesn't have the sense not to talk back to her, so he gets punished right away, even though he has better luck than the other kids when trying to answer her academic questions. In his conversation with the Trunchbull, Nigel tells us a little bit about what it's like to be one of Miss Honey's students. Apart from Matilda, he might be the bravest of all the kids in the class, and one of the liveliest. After the Trunchbull faints, he pours a pitcher of water on her head. To which we say, you go Nigel.
Rupert comes to the Trunchbull's attention because he has very nice, long blonde hair. The Trunchbull hates this almost as much as she hates pigtails (she sure seems to care about hairstyles). When Rupert gets her academic questions wrong, she picks him up by the hair and shakes him about. Rupert's not as brave as Nigel and wilts under the pressure.
The next little boy to come forward, though, Eric, is pretty brave. He brings the Trunchbull's wrath on himself by stepping up and asking her if she was ever a little kid, too. Not the smartest thing Eric's ever done, and he pays for it, too. When he gets one of the Trunchbull's questions wrong, she picks him up by the ears and waves him around. Like Rupert, he has to recite the correct answer before getting released. So these three boys all tangle with the Trunchbull the first time she ever visits their class.
And the Trunchbull catches Wilfred the following week. The Trunchbull is in the middle of waving him around the room—by an ankle—when the chalk starts writing on the blackboard, all by itself.