Jolly Foul Play by Robin Stevens

A new school year begins in Deepdean, and nothing is the same: the new head girl Elizabeth Hurst and five supervisory students spread fear and terror among the younger girls. But on Bonfire Night, Elizabeth is suddenly found dead on the hockey field. It quickly becomes clear to Hazel and Daisy: It wasn't an accident and the murder must have been committed by one of the students. Many girls in Deepdean would have had a motive to hate Elizabeth, but which one is playing foul play here? Could the murder be connected to the many secrets revealed on little pieces of paper at school? The Wells & Wong detective agency begins their investigation and has to overcome many obstacles - not least an argument that puts their own friendship to the test...