This is a bit of a tricky one. Agnes is our narrator, and we hear events recounted through her perception, but she speaks for the group. She lets us see inside her own life a few times; we learn that she wants her BFF Ursula-Marie's hair and wardrobe, but she spends a lot of time saying "we" instead of "I." Sure, she's pretty freaked out at the thought that Pierre Anthon could be right, but she nestles her doubts firmly inside those of the group. So it's not "I'm doubting" so much as "We're doubting," and it's not so much "I got in trouble," as "we did."
Take this moment, for example: "Sofie screamed and screamed. She screamed so loud and so piercingly it made our ears ring and hurt right to the bone" (23.16). There's no way Agnes could know if everyone else's ears were ringing and their bones were hurting, but she's become so consumed by the group that she can't even use first-person pronouns when talking about her own physical sensations.
By having Agnes narrate the book, Teller is showing us Pierre Anthon through the eyes of his peers. If Pierre Anthon told us this story, it probably wouldn't be much of a story. We'd just get, "Everything is meaningless. I've been sitting in a tree for three months thinking how stupid everybody is. The plums are good, though."
But through Agnes, we see how Pierre Anthon's eccentric behavior affects other people, and the lengths those people go to in an attempt to change his behavior. The plot, the story, the action, take place in Agnes'sworld, the world of Class 7A. After all, nobody's chopping off any fingers up in that tree.