Because of Anya — Margaret Peterson Haddix

Ten-year-old girls don't wear wigs.

So why is Anya wearing one? That's what Keely wants to know. But when Anya's wig falls off in front of the whole class, Keely realizes what she really wants is to help Anya, even though she's not sure how--and even though it means she'll have to do something she's afraid of: stand up to her friends.
     As for Anya, she just wants her hair to grow back, but no one can tell her whether it ever will. How can she learn to accept her disease when she can't even look in the mirror?