Why is Meg unhappy at school in A Wrinkle in Time?

Meg is
unhappy at school in for several reasons. First, she feels she doesn't fit
in. Her hair doesn't look right, she wears braces on her teeth, she isn't athletic and
"normal" like her twin brothers, Sandy and Dennys, and she doesn't do well
academically. One of her teachers has just told her that she will be held back if...