Murder Most Unladylike. A Murder Most Unladylike Mystery, Paperback/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.