The bacteria was found in a disused shower block at Hillcroft Special School, after planned water testing across the North Eastern Education and Library Board area.
The strain of bacteria can cause Legionnaire's Disease, a potentially fatal form of pneumonia.
A NEELB spokesperson said the school was closed on Friday to allow further tests to be carried out and will remain closed until at least Monday 11 February.
"We would like to reassure anyone who may be concerned that the area where the bacterium was found has not been in use by the pupils or staff at the school," a board spokesperson said.
"The parents of pupils attending Hillcroft Special School have been notified by the Principal and we will continue to work closely with the school, the Public Health Agency and the Health & Safety Executive to resolve this issue as quickly as possible".