How is Nora acting in the book and why? Why does she want to show her parents bad grades?...
Nora is an exceptionally bright kid. She is genius level smart. No exaggeration there either. She really is a certifiable genius, and she knows it. Nora doesn't want to stand out because of it though, so she intentionally gets grades lower than her abilities.
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.
She wants to be normal and be treated as such. Nora feels that if she shows her parents her bad grades, then her parents won't try to push her to be exceptional. In fact, Nora is making her life a bit harder with her parents, because her parents keep punishing her because of the bad grades.
If I had to summarize how Nora is acting into one word, I would choose stubborn. It's probably more work for Nora to get bad grades and get punished by her parents than it is for her to work to her potential; however, Nora's number one goal is not comfort. Her end goal is fitting in.