The suicide cluster was followed by a number of other traumatic events - including more suicides, attempted suicides and a number of other deaths within the school community - over more than a decade.
The couple said they were excepted to be constantly on-call, and had to deal with at-risk students without proper support or facilities.
Employment Relations Authority member James Crichton found the events provided "a harrowing context for employment".
However, he dismissed the couple's personal grievance claim because it was not raised within the statutory 90 day timeframe.
Mr Crichton said there was little doubt the earlier traumatic events had caused the couple's illness, rather than more recent events, including interpersonal and training issues, which were raised within time.
He said there was nothing to link the later events to the earlier traumatic events.
"The authority must conclude therefore that the tragic events of the earlier part of the employment are not up for consideration as evidence to support the personal grievance claim."
Mr Crichton also dismissed a counter-claim by the school. Costs were reserved.