"Mummy" by Caroline B. Cooney
Emlyn is a high school student, and the "good one" in her family; she does well in school, participates in extracurricular activities, and never causes trouble to her parents. But deep down inside, she hatches secret and unfulfilled plots to rob banks, smuggle things into airports, and a thousand other crimes.
Four classmates think goody-goody Emlyn could provide the perfect cover for their senior prank-stealing the Egyptian mummy from the nearby museum and suspending it from the bell tower of their school. Emlyn is intrigued at the opportunity to finally put one of her plans into effect. But she finds that when you have a priceless bit of history in your possession, it's hard to stop yourself from wanting to protect it at all costs.
Emlyn is an interesting and imperfect character, a nice girl who dreams of doing bad things, but then isn't really prepared to do harm. Her parents are completely absent from this story, which leads to an interesting topic for discussion: whether kids are more likely to get into trouble when their parents are uninvolved in their lives.