One difference is in the character portrayals. In the movie, the characters might be more dramatized for the visual medium. Another is the pacing. The movie may have adjusted the pacing for better cinematic flow compared to the novel.
The movie might simplify some of the complex ideas in the novel to reach a wider audience. Also, there could be changes in the sub - plots. For instance, certain side stories in the novel might be given less emphasis or altered in the movie to focus more on the main story of friendship and the educational system.