How can I help my sister who is being picked on at school?

My sister is pretty different from all the other high school girls. She wears her hat every single day, she doesn't wear makeup, and she's pretty nerdy. She reads old books over and over again, like Pride and Prejudice and The Great Gatsby. She always brings a book to school. But then she gets picked on. She had to take her hat off for PE, and she locked it in her PE locker, but she said that some girls picked the lock on her locker and stole her hat, and she saw them making fun of it by calling it "stupid," "geeky," and a "cap of shame." Also, they took one of her books and apparently tried to play basketball with it. I'm not sure what exactly went on, but her book is definitely beat up, which is odd because her books are her most prized possessions, and she takes really good care of them. Now, she's always telling me that she wants to stay home from school (I'm her guardian). Finals are coming up, and I don't want her to do badly on them just because she wants to avoid the bullies at school. But I also don't want her to feel unsafe at school. She wouldn't rat out the girls who are picking on her, and she's scared of being picked on more if she stands up to them. What should I do to help her?