The blaze on the doorstep of the high school was discovered at 10.45 pm on Saturday.
Two men were seen around the boiler room area of the school and police believe they broke in and took out items to start the fire.
The pair were seen leaving the area an hour later at about 11.45 pm.
Constable Ian Dunn said: “The only description we have of the males is that they were wearing dark clothing and hooded jackets.
“It doesn’t need to be explained the absolute recklessness of such an act and should the fire have taken hold of the building then it could have been catastrophic. As it was, the door to the boiler room was damaged by the fire.”
Anyone with information which might help catch those responsible is asked to call officers on 101.
