The book's beauty lies in simplicity which is praised throughout the book. Even if it's short and with a simple plot, it gives insight to the teachers' thoughts and the "troubled" kids' inner world and struggles. For adult readers it may seem underwhelming but it's quite didactic for its target audience. It addresses issues such as stigma, marginalization, social rejection, racism , immigration and feminism. So the rating depends entirely on perspective. If we look at it as "did it serve it's purpose" then it's a pretty good book, but reading it objectively as an adult it's a three star book.
Plot : May feel underwhelming but served its purpose. Characters: Kinda stereotypical ( I'm guessing it's because they had specific roles in order to bring out a specific message) and kinda hard to care in depth about outside the purpose they serve in the story.