Half bad

Half Bad by Sally Green is a breathtaking debut novel about one boy's struggle for survival in a hidden society of witches.

You can't read, can't write, but you heal fast, even for a witch.
You get sick if you stay indoors after dark.
You hate White Witches but love Annalise, who is one.
You've been kept in a cage since you were fourteen.
All you've got to do is escape and find Mercury, the Black Witch who eats boys. And do that before your seventeenth birthday.

Easy.