Crews were called to empty school, near Whitcliffe Lane, at about 6pm yesterday (Saturday).
The classroom was severely damaged by the fire and extra crews were called in from Harrogate, Boroughbridge and Kanresborough to extinguish the flames, a spokesman for North Yorkshire Fire and Rescue Service said.
"The cause of the fire is believed to be deliberate involving a small amount of rubbish set alight inside an old classroom," the spokesman added.
It took two hours for the firefighters to extinguish the blaze.
Crews are set to return to the site today (Sunday).