They do it with mirrors / Christie, Agatha

Miss Jane Marple and Inspector Curry of Scotland Yard are both investigating a vicious, cold-blooded murder at Stoneygates, a home for juvenile delinquents.

Miss Marple's friend is being poisoned so she goes to Stony gates and finds more intrigue.

Miss Jane Marple is called out to a country estate to check on her rich friend Carrie, who has turned her mansion into a reform school for juvenile delinquents, and finds herself investigating a strange murder that points toward the workings of magic.