To write a short summary of Andrew Clements' The Report Card, focus on explaining who the main characters are and what the main events are. The main events in any book will always be related to the conflict, climax, and resolution.
Andrew Clements' The Report Card is about Nora Rowley, a girl born a genius who decided the best thing to do was to hide her intelligence in order to seem like everyone else. She does so because she observes that people get jealous of others who are different from them and because, when she first discovered she wasn't normal, she felt like she was being pushed to perform her abilities, which she didn't like. After that, she very intentionally behaved in ways that made her seem like everyone else.
The conflict develops when she begins noticing her best friend Stephen is beginning to feel stupid because he felt so pressured to do well during the Connecticut Mastery Testing that he actually did very badly. After that, Nora developed a plan to show that neither bad nor good grades are true reflections of intelligence, since she intentionally earns average grades all the time but has a genius intelligence level. She carries out her plan by trying to intentionally earn all Ds on her fifth-grade report card.
The climax begins to develop as it becomes known that Nora is actually a genius who has been hiding her abilities all these years. Since her plan to expose the meaninglessness of grades was foiled once people saw her for the genius she truly is, she and Stephen develop another plan to encourage the whole class to earn nothing but zeros in order to protest against the weaknesses of the education system. In Nora's mind, the education system is weak because the lessons only encourage students to memorize, not think, while grades encourage needless competition. The climax occurs once it begins to look like Nora and Stephen will get into serious trouble for pursuing the plan.
As a resolution, Nora realizes that, though she was right to think poorly of the education and grading system, she was wrong to try and tackle changing the system all on her own. She further realizes that her actions caused more harm than good and apologizes to her classmates, encouraging them to continue to work hard.