Jolly Foul Play by Robin Stevens

Things had changed a Deepdean. All of the rules had bent, and the power had moved. Elizabeth Hurst's reign was over, and none of us knew what would happen next. Daisy Wells and Hazel Wong have returned to Deepdean for a new school term, but nothing is the same. There's a new Head Girl, Elizabeth Hurst, and a team of prefects - and these bullying Big Girls are certainly not good eggs. Tensions are running high. Then, after the fireworks display on Bonfire Night, Elizabeth is found - murdered. Who might have committed such foul play? And with Deepdean running riot, and their own friendship falling apart, can the Detective Society solve the case?