Cat Among the Pigeons book by Agatha Christie

Unpleasant things are going on in an exclusive school for girls – things like murder… Late one night, two teachers investigate a mysterious flashing light in the sports pavilion, while the rest of the school sleeps. There, among the lacrosse sticks, they stumble upon the body of the unpopular games mistress – shot through the heart from point blank range. The school is thrown into chaos when the ‘cat’ strikes again. Unfortunately, schoolgirl Julia Upjohn knows too much. In particular, she knows that without Hercule Poirot’s help, she will be the next victim…