Amanda is in the wrong place at the wrong time, with the wrong hairstyle. She appears in just one scene, but it's an important one—it's the moment when Lavender and Matilda see the Trunchbull in the flesh for the first time, and learn what the monstrous headmistress is really capable of.
Amanda makes the mistake of wearing pretty braids that show off her hair to school. As Hortensia tells Matilda and Lavender, the Trunchbull finds braids gross. So when the Trunchbull sees Amanda's hairstyle, she goes bananas. The Trunchbull spots Amanda on the playground and charges at her.
It ends pretty badly for Amanda. After telling the girl she has to chop off all her hair before she comes back to school, the Trunchbull picks her up by the hair, whizzes her around at great speed, and throws her across the grounds, where she lands in a field. In real life, this would've probably killed her—actually, in real life, it would've probably been impossible for the Trunchbull to swing her around by the hair—but since we're in a Roald Dahl book, Amanda just bounces on the ground, landing only a little worse for the wear.
This horrifying scene with Amanda teaches Matilda something important about the Trunchbull—that she's a nasty, horrible brute of a woman. Afterwards, Matilda's got an important key to the Trunchbull's personality: as she tells Lavender, "'Never do anything by halves if you want to get away with it. Be outrageous. Go the whole hog. Make sure everything you do is so completely crazy it's unbelievable. No parent is going to believe this pigtail story, not in a million years. Mine wouldn't. They'd call me a liar'" (11.4-6).
So while we're imagining Amanda's scalp is pretty sore, we're glad Matilda was able to learn something from the scene.