I think it was made in the 70's - and could have had a remake later
The movie was about a boarding school, the a wife and a mistress murder the husband, headmaster and supposedly throw him in an abandoned pool/pond on the property - one of them tries to drive the other one insane by playing piano music that only the husband knew, by draining the pool only to find out he's not there, by leaving phone calls and other indications that he is still alive...

Loved the movie - was trying to share it with my kids and can't remember the name

Any idea what I am talking about

There are a number of versions, with the original being "Les Diaboliques" aka "Diabolique" (1955).

Here's a list of remakes:
The Corpse (1971)

Reflections of Murder (1974 TV movie) ~ likely the one you are seeking.
http://www.imdb.com/title/tt0072070/

House of Secrets (1993 TV movie)

Diabolique (1996)