Fire crews from across the district were called to battle the blaze at around 6pm on Saturday, and remained at the scene for two and a half hours. The fire caused severe damage to a classroom.
The Chairman of Ripon Civic Society, David Winpenny, said the incident highlights a growing need to ensure that buildings have the best levels of protection.
Mr Winpenny said: "The fire at the Cathedral Choir School underlines the vulnerability of Ripon’s historic buildings; too many are left without adequate protection when their function has ended and while new uses are being sought.
"Vandalism and arson are all too likely where the future of a building is in doubt. Ripon Civic Society has been anxious about the Choir School main building - once the grandstand for the former racecourse, for some time, and is still concerned about the future of the Old Lecture Block – formerly the Girls' High School, which continues to deteriorate.
"We look to the owners of all properties to ensure they are secure - and to Harrogate Borough Council, to use their powers to see that this is done."
A North Yorkshire Fire and Rescue spokesperson said: "The cause of the fire is believed to be deliberate involving a small amount of rubbish set alight inside an old classroom.
"The classroom was severely damaged by fire and crews requested the assistance of the aerial ladder platform from Harrogate and extra support from Boroughbridge and Knaresborough crews."