ALS is also referred to as motor neurone disease (MND).
Mr Dempsey, the Jeppe High School for Boys headmaster, was challenged by the headmaster of King Edward VII School, Mr David Lovatt.
A special assembly was called and Mr Dempsey then nominated Dr Pierre Edwards of Affies, Mr Chris Luman of Maritzburg College and Mr Paul McAvoy of Northwood before having three buckets of ice-water poured over his head.
Watch the video here
