West Yorkshire Fire and Rescue Service said they have used a special tool, which can pump high volumes of water great distances, to help tackle the flames at the former Elmete Wood School in Roundhay.
Crews were called to the scene at around 6pm on Saturday, March 6, and flames could be seen from up to a mile and a half away.
Watch commander Tim Pallister said the roof of the school has been destroyed and the building will now likely be demolished.
He said: "The fire's mainly out. There's a few pockets of fire left that we are currently dealing with. We've got three fire engines in attendance, also got an aerial (ladder platform) to fight the fire from above. We have our fire investigation department here as well, as well as the police.
"We're using a specialist pump called a 'high volume pump' and that's supplying water from Roundhay Park from the lake. It's a pump that can pump massive amounts of water from quite a distance.
"Most of the roof has gone now. The fire is actually on the floor - it looks like a concrete floor - so it's just pockets of debris that are about. The structural engineer has just been and we're not allowed to enter the building now because it's an unsafe structure so all firefighting is from outside."
Asked how long the crews would be on the scene for, watch commander Pallister added: "It's probably into tomorrow (March 8). It will be a structure that will end up more than likely being demolished but as of yet, we're not sure when that's going to happen."
Nearby residents were advised last night to keep their windows closed due to the smoke, and watch commander Pallister said while asbestos has been found at the site, it has been dealt with by the fire service.
"I'd keep windows closed," he added. "There's a chance of asbestos in the building but we've dealt with it sufficiently by putting water on it so it dampens the dust down, which should keep it relatively safe."