The Shadow Society by Marie Rutkoski

Darcy Jones was five when she was abandoned at a firehouse in Chicago and has been flying through foster parents since.  At the age of sixteen she is finally going to the same school for a second year in a row.  This new year has been great up until she kisses her crush, gets arrested and her foster mother throws a kitchen knife at her all in the same day.  Darcy responds by vanishing.  And by doing so she finally starts to find some answers.  Fantastic book.