The ceiling has now come down on parts of the North West Community Campus in Lochside which is due to open next month after a £28m construction programme.
Worried parent have been alarmed by social media pictures which suggest the ceiling has collapsed.
But in a statement Graham said: “During standard testing toward the completion stage of construction, a leak occurred within the staffroom area.
“This testing was carried out in the evening and in a controlled environment supervised by experienced members of staff.
“The matter was identified and isolated and acted upon immediately. The weight of water over a period of time saturated ceiling tiles and caused sagging and superficial damage.
“It is not uncommon for issues to be identified during testing so that we can rectify them.
“We can confirm that full quality assurance procedures have been adhered to on this project.
“An investigation has been instructed to ensure full understanding of the cause of the leak.
“The campus will be completed as per the agreed programme with the authority, creating state-of-the-art facilities of a high quality for north-west Dumfries and Graham are proud in delivering this project.”
Work started on the £28m project in 2016 as a replacement for the current Lochside and St Ninian’s primary schools, Langlands special school and Maxwelltown High School.
Dumfries and Galloway Council: said: “Dumfries and Galloway Council were informed that there had been a leakage during testing. Grahams have given us their assurance regarding the issue being fully rectified and all necessary checks and due diligence being carried out.
“At the moment, the facility is still under the ownership of Graham Construction. The Council will not take ownership of the building until all construction works have been completed to the satisfaction of the Council.
“North West Community Campus will be a state of the art, modern learning environment for use by pupils and the community. We look forward to opening our doors to welcome pupils when the term starts on 21 August.”