The building has been empty for a number of years.
An SFRS spokesperson said: “We were alerted at 7.28pm on Saturday August 15 to reports that a fire had taken hold within a building.
“Operations control has mobilised six fire appliances to Old Viewforth High School in Kirkcaldy and crews are presently working to contain and extinguish the fire.”
Police are urging people to avoid the area and close their windows as smoke billows over the area.
Road closures have been put in place while emergency teams tackle the fire.