Murder Most Unladylike by Robin Stevens

When Daisy Wells and Hazel Wong set up their very own secret detective agency at Deepdean School for Girls, they struggle to find any truly exciting mysteries to investigate. (Unless you count the case of Lavinia’s missing tie. Which they don’t.)

Then Hazel discovers the Science Mistress, Miss Bell, lying dead in the Gym. She assumes it was a terrible accident – but when she and Daisy return five minutes later, the body has disappeared. Now Hazel and Daisy not only have a murder to solve: they have to prove one happened in the first place.