Do-Over (by Christine Hurley Deriso)

Then she is visited by the ghost of her mother, who gives her a locket with a special power -- with it she can do the last ten seconds over whenever she wants. Using it she tries out several methods for making herself popular, but finds it's harder than she thinks.

This seems like such an intriguing idea: a nice kid learning to navigate the treacherous waters of middle school by getting to fix her mistakes repeatedly until she gets it right. But the main character is supposed to become more appealing, not less so. Given this wonderful power, and advice from her ghost-mom to be true to herself, Elsa immediately starts becoming what she supposedly hates.

She cheats to prove she's smart, then pretends to be stupid, cuts off her unpopular friend, sucks up to the mean popular girl, passes along vicious gossip with barely a qualm, and plays nasty pranks for revenge. Of course this all backfires and she learns better, and then her problems magically disappear. But it seems like she always had a popularity princess inside her just waiting to get out. This book is enjoyable enough, but with this premise it could have been much more.