August tells the reader that the night before, he Googled what "like a lamb to the slaughter" meant. He feels exactly like that when Ms. Petosa calls on him and asks him to speak up. When he introduces himself to the class, he talks about Via and Daisy as his interesting facts, and then Julian asks him if the little braid in his hair is a Padawan braid. Julian takes it upon himself to explain that a Padawan is a Jedi apprentice from Star Wars, and Ms. Petosa kindly asks August if he likes Star Wars. August wants to disappear under his desk when Julian asks if he likes the character Darth Sidious—a character whose face gets burned and melts in one movie. Mrs. Petosa doesn't get the reference, but August sneaks a peek at Julian and can tell that he meant to be hurtful.
It's clear to August that Julian is trying to bully him by bringing up Darth Sidious, and it's telling that Julian is successful. As a teacher, Ms. Petosa would be required to shut down that kind of bullying if she knew it was happening, but she is oblivious to it. This illustrates how bullying can happen anywhere—even under the nose of kind, engaged, and well-meaning teachers. This begins to show August that he'll have to fend for himself at school, which in turn indicates that the bullying battle will be a strictly among kids and carefully concealed from the adults.