I Know You Did It
I Know You Did It by Sue Wallman
There's a killer at Ruby's new school.
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.
Or, she might be next.