Hillcroft Special School was shut last week as a precaution after the bacteria, which can cause Legionnaire's disease, was found in a disused shower block.
Tests were carried out on Monday, however the North Eastern Education and Library Board says it will not have definitive results until next Friday.
It is hoped the school will be opened again by 25 February, after the half term break.
"Staff continued to work throughout the period of school closure, including collating work packs for the children and delivering them out to the pupils at home, engaging in staff development, attending training and undertaking school visits," a statement said.
"The board appreciates that this is a very stressful time for the pupils, parents and school Principal. We fully appreciate the co-operation of the parents and the on-going work of the Principal and staff at this difficult time".