The main character in The Report Card is Nora. She is the one who calls the education system into question in this book (which is the whole point of the story, similar to Clements' other books such as Frindle).
Nora allows her grades to drop purposefully so that she can show the negative influence of grades on students. In fifth grade, however, Nora just wishes she didn't get so much attention from her parents about these "stupid" grades. The parents force the kids to read their report cards out loud. Here is Nora's reaction:
They’re all based on a bunch of stupid information that anybody with half a brain can memorize. Tests and grades and all of it—it’s all...just stupid.
As we can see from the quote above, Nora is not ready to reveal what she is trying to prove about grades and gets in trouble for the evidence. Meanwhile, Nora is spending her time doing graduate-level work in the library unbeknownst to everyone except the librarian!
It’s perfectly clear to me now that you are not a below-average student, or even an average student. Far from it. And you’ve been hiding that from me and everyone else at school.
Truth be told: Nora is a genius. However, Nora is very careful not to let anyone know how smart she is (by always getting Ds on her report card) in order to prove her point about grading.
The irony is that, at the end of the book, Nora can't outsmart the IQ test and proves to everyone just how much of a genius she is.