Diabolique (movie, 1996)

The wife and mistress of a cruel school master collaborate in a carefully planned and executed attempt to murder him. The plan goes well until the body, which has been strategically dumped, disappears. The strain starts to tell on the two women as a retired police investigator who is looking into the disappearance on a whim begins to think that they know more than they are telling, and their mental state is not helped when their victim is seen, apparently alive and well by one of the pupils.