Don't Look Behind You by Lois Duncan

Overnight, April has to pack up her whole life and leave her boyfriend behind with no explanation to him when she finds out her father has been working undercover for the FBI. Because her father's testimony has convicted a notorious drug dealer, April and her family have to change their names, their appearances, and move into the Federal Witness Security Program. April finds it harder to move on from her old life though which results in dire consequences.