When cell phones are banned at their school, students start communicating through sticky notes left all over the school, triggering a wave of bullying activities in the wake of a new girl's arrival.

Riley Flynn is alone. It feels like she’s been on her own since sixth grade, when her best friend, Emily, ditched her for the cool girls. Girls who don’t like Riley. Girls who decide one day to lock her in the science closet after hours, after everyone else has gone home. When Riley is finally able to escape, however, she finds that her horror story is only just beginning. All the school doors are locked, the windows won’t budge, the phones...