Meg's having a pretty hard time of it at school. From every angle, it seems that she's being constantly criticized. Her teachers accuse her of being a bad student, and because of this she ends up being dropped to a lower grade. If this weren't bad enough, she's accused of babyish behavior by some of her fellow students when she tries to act all tough.
As well as being attacked herself, Meg also has to listen to criticism and name-calling of her brother Charles Wallace. But Meg doesn't take any of this lying down. She's fiercely loyal to her brother and often gets into fistfights with those who poke fun at him.
One could argue that such behavior is brave, though far from ideal. Meg means well; she only wants to protect her brother. But as the story progresses, she learns that there are much better ways to display courage and that violence is unnecessary to this end.
In searching for her missing father, Meg shows remarkable bravery, not least in her encounters with the evil superbrain IT. In saving Charles Wallace from IT's evil clutches, she also learns that love, not violence, is what gives you courage, what gives you the strength to face up to life's many adversities.