Why does Meg act dumb in school?

Meg is totally lacking in self-confidence and tends to act in the same way she sees herself. Far from dumb, her own vision of herself keeps her from excelling, and the difference between her and her classmates creates a huge void. Yes, Meg's classmates aren't crazy about her.... but her behavior certainly doesn't help her cause. None-the-less, we also have to see Meg as the girl she is..... she's in crisis. Her father has disappeared, and she knows he's a source of gossip and entertainment..... she also loves and misses him.