Arsenic for Tea by Robin Stevens

1934. When Daisy Wells and Hazel Wong set up a secret detective agency at Deepdean School for Girls, they struggle to find a truly exciting mystery to investigate. (Unless you count the case of Lavinia’s missing tie. Which they don’t.)

But then Hazel discovers the body of the Science Mistress, Miss Bell – but when she and Daisy return five minutes later, the body has disappeared. Now the girls have to solve a murder, and prove a murder has happened in the first place before the killer strikes again (and before the police can get there first, naturally), but will they succeed?