Crews were called to a fire at Aspin Park Primary School on the afternoon of July 22.
Police are investigating the fire which is believed to have been started deliberately.
North Yorkshire Police's Community Messaging Service said: "Unknown individuals have climbed onto the roof of the school and have forced open a window before dropping a lit item into the classroom.
"Police are appealing for anyone who may have information in relation to this. If you have any information that can assist please contact 101 and quote reference 12180133821."