I know you did it
Sue Wallman
On her first day at a new school, Ruby finds a note in her locker saying "I KNOW YOU DID IT". She's terrified that someone has found out she was responsible for the death of a girl called Hannah in a playground when they were both toddlers - a secret that has haunted her for years.
When first one, then two students are found dead, it looks like Ruby is being framed by another killer.