They do it with mirrors - Agatha Christie

When Jane goes to stay with her friend Carrie-Louise at her home she soon begins to realise that all is not well there. Not only is this the family home but it is also a rehabilitation centre for delinquent boys and there are definitely some odd characters living there. Then an attempt is made on the life of Carrie-Louise's husband, her step-son is murdered and someone tries to poison Carrie-Louise....