Glasgow's Mackintosh building will definitely be rebuilt, according to Muriel Gray, chairwoman of the Glasgow School of Art's board.
The building was gutted by fire in June, following another blaze in 2014.
The future of the GSA has faced speculation that it could be demolished or turned into a museum.
But Ms Gray, a former student at the school, told BBC Scotland it would be rebuilt as a working art school, saying that was "non-negotiable".
She told BBC Radio's Good Morning Scotland programme the £100m cost that had been reported was "plucked out of the air", saying the art school had "absolutely no idea until we know the scale of damage".
The former art school student said there would be full transparency and that the cost should not impact on the taxpayer as some of the money raised in 2014 remained.
She also acknowledged that it could take up to 10 years for students to be able to use the building again which she said was the "biggest tragedy".
Ms Gray told the programme the decision to rebuild the Mackintosh building was made after the fire four years ago and that position had not changed.
"The board of governors were completely unanimous about this and it's not up for discussion," she said.
"The debate now is how we fit in to the whole tragedy that's happened to the entire Garnethill community and the Sauchiehall street refurbishing and how we do that as a working art school."
Responding to comments about delays in getting people in the area back into their homes and businesses, Ms Gray said: "We have been removing the unsafe masonry, declared unsafe by building control at the council, as fast as we possibly could, in order to allow people back.
"We've been absolutely working so hard with everyone round about us to try and make things happen quickly enough to get people back into their homes, to try and help businesses. and liaising with all the people affected."
She added: "We were months away from just having the best, biggest party ever that was going to totally regenerate that part of Sauchiehall Street and Glasgow - the council building this beautiful regeneration plan - and it's put back again.
"You can either sit in a corner in a foetal position and weep about that or you can face up to the reality and go 'right, setback again, we are just going to come back and do this brilliantly'.
"There is absolutely no way it wouldn't be a working art school. It's non-negotiable.
"This has all been really exciting because we're actually using the blueprints from the interior of Mackintosh's original plan for the building."
Francis McKee, director of the Glasgow's Centre for Contemporary Arts (CCA), which has been inaccessible since the fire, said a toilet in the building had been leaking for three months which had brought down the ceiling of a gallery below.
Gallery work 'lost'
He told the programme: "For the entire time, most of the building has been outside the cordon. That lack of access, when it could have been - through common sense - given, has meant we've lost the ceiling of a gallery and the artwork in the gallery.
"That didn't have to happen. A lot of those organisations can't go anywhere else because we offer space for free. It's not about the CCA, its about the 366 partners with the CCA. We don't have the money to move things anywhere else.
"It's a very complex business and it's being treated like it's a small shop."
Mr McKee said the dates he had been given for moving back in kept changing.
Colin Edgar, head of communications at Glasgow City Council, said: "The way you would traditionally do this is you would come up with a programme, you would test it, do it, then once it's done you would start going to people 'right, its safe, now you can get back in'.
"What we're trying to do is say to people 'we think you'll get in on the date and you can start to make plans but you have to know that that date's at risk'.
"Everyone needs to work quickly together to get Francis (McKee) back in as quickly as possible but then we need to look at the future of Sauchiehall Street, which we have already started to do."