"It's getting worse and worse," said Emlyn Firth, a graphic design teacher at the school. "There's now speculation from the fire brigade that they might have to pull it down, that it's going to be unsafe."
He added: "Looking at it right now it's hard to see how that's going to be structurally sound."
"No one really knows because they're still battling the blaze," he said.
Speaking from the scene, Firth told Dezeen that the fire had "ripped through" the west side of the building.
"There's flames shooting out of the library windows and the roof," he said. "There's a huge plume of smoke rising into the air."
Firth, who has been teaching at the school for more than six years, said that people were crying in the street.
"For a lot of people it seems like a nightmare. It feels like this is one of those crazy dreams," he said. "It's a place that people come to from all around the world and they come because of the building. Students from Japan, China, they want to come and study here. It has a magnetic draw. The art school is kind of like a life force for the city."
In an official statement, the Scottish Fire and Rescue service said that it had been fighting the fire from inside and outside the building but was not yet issuing any further information.
Chief officer Alasdair Hay said: "This is likely to be a protracted incident and crews have been working extremely hard to tackle what is clearly a very significant fire."
"The priority throughout this operation has been to protect life but salvage operations are also underway."
Update 5.30pm, Friday 23 May:
In a statement published on the Glasgow School of Art press site, Muriel Gray, chair of the board of trustees of the Glasgow School of Art, said: "today is a really black one for the GSA, but I cannot thank the fire brigade enough for the speed with which they came and their commitment to contain and extinguish the fire. Fortunately there have been no fatalities or injuries."
"I am so proud of the staff and students and how everyone has pulled together. We are thankful to all the Glaswegians who turned up to comfort students and to friends from across the world for their messages of support."
The fire is thought to have been started by an exploding projector that set light to expandable foam used in an art installation. For more information see our previous story.
Top image of Glasgow School of Art by Finlay McWalter.