First the easy stuff. Nora is an eleven-year-old girl. She is short with red-blond hair. She is in fifth grade. Oh, and she's a certified genius, though nobody except her knows it. She fakes all of her test scores and intentionally does poorly to mask her high levels of intelligence. That tells you a lot about this little girl. It tells you that she is very aware of her talents, and she is very aware of the fact that she will likely stand out because of her intelligence. For some people that would be okay. For Nora, though, she prefers to stay secretive.
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.
That tells the reader that she is self-conscious and maybe even embarrassed that she is "different." She deeply desires to be "normal." I would call her stubborn too. Her parents even ground her for bad grades, but Nora continues to go about her plan despite being punished for it. However, despite intentionally getting bad grades, she likes learning. She taught herself Spanish at a young age and loves a wide range of subjects.