Fall Into Darkness by Christopher Pike

Sharon McKay was going to head off to Juilliard next year, but when she's accused of ***ing her best friend, Anne McFarland, everything changes. While on a camping trips with friends, Sharon and Anne went to the top of the cliff. All their friends heard was Anne scream, "Don't!" before she toppled over the ledge and into the water; her body was never found. Sharon knows she didn't kill her best friend, but not even she thinks that Anne was suicidal, which leaves her as the only suspect. Somehow, Sharon must prove her innocence.