Bad grades aren't always a sign that a student has suffered a trauma. In Melinda's case, it's striking because, according to Mom and Dad, she used to get good grades. Along with the host of other negative signs Melinda is giving off, bad grades is a sign that something is seriously wrong with Melinda. Unfortunately, adults just seem to give her a hard time about it and don't pick up on the truth.
We do, though. At the end of every part of the novel (the "marking periods") we learn Melinda's grades. How well she's doing in school also tells us how well she's doing internally.