After an inspection of the Charles Rennie Mackintosh building today, the school revealed that the majority of the structure was "intact".
Broadcaster Muriel Gray, the art school's chairwoman, said the institution's archives had also been saved.
She said: "Bad news first is that we have lost the iconic and unique Mackintosh library. This is an enormous blow and we are understandably devastated.
"But the most amazing, almost miraculous news is that the majority of the building is still intact.
"Due to one of the most astonishingly intelligent and professional pieces of strategy by the fire services, they succeeded in protecting the vast majority of the building, apparently by forming a human wall of firefighters up the west end of the main staircase and containing the fire."
Smoke could still be seen rising from the charred windows of the school this morning - 24 hours after the flames took hold.
No-one was injured in the fire but art lovers, architects and famous former students spoke of their sorrow at seeing the building in flames.
The fire service has yet to confirm the cause of the blaze, which some students have suggested could have started in the basement when a spark from a projector caught a piece of foam.
As well as housing one of Europe's leading art schools, the listed Mackintosh-designed building is a tourist attraction in its own right.
Completed at the turn of the 20th century, it was voted as the best building of the past 175 years in a poll by the Royal Institute of British Architects (Riba).
Ms Gray said the damage was "considerably less" than feared and although students have lost some or all of their work, many others had theirs preserved.
She said staff and curators would be able to enter the building to assess what could be salvaged in the next few days.
"The joy that our archives are safe combines with the delight in seeing most of our beloved building bruised and battered but most certainly not destroyed.
"As for the library, Mackintosh was not famous for working in precious materials. It was his vision that was precious and we are confident that we can recreate what was lost as faithfully as possible.
"Our main concern right now is the welfare of the students and the impending graduation and everyone is working hard together to achieve the best outcome for all."
She thanked the public for the "warmth of support" and said she had "run out of words" with which to thank firefighters.
"But the school has most certainly gained a new gallery of heroes" she added.
Around 200 firefighters have been involved in the operation, which at its height saw 16 appliances at the scene, the Scottish Fire and Rescue Service said.
Chief officer Alasdair Hay said: "The firefighters who responded to this incident demonstrated incredible courage, skill and determination to prevent the complete destruction of this iconic building and its contents.
"Choosing to fight a fire of this scale from inside the building is a risk for firefighters and requires the highest standards of professionalism.
"Those involved in this incident were predominantly drawn from greater Glasgow and they were certainly very aware of the importance of the Mackintosh to the city. We have all been conscious of the fact this is also a building that houses the hard work of Glasgow School of Art students, especially at this time of year.
"Operating in extremely dangerous, challenging conditions our crews conducted highly aggressive firefighting operations and implemented an effective and informed salvage plan.
"By working very closely with staff from the art school, we were able to identify items and target our efforts to recover items of great importance and save everything that could possibly be saved."
He said efforts to stop the blaze engulfing the entire building were helped by early decisions and aggressive fire fighting.
A joint investigation with Police Scotland will get under way into the circumstances surrounding the fire.