There is significant disruption to neighbouring roads due to the scale of the fire at the old Overdale School buildings in Eastfield.
No injuries have been reported.
Emergency services are still on the scene and Police are appealing for information about three suspects seen at the site.
All three are teenage boys.
The first is described as thin, white, around 15 years old and had a bowl haircut. He was wearing a black jacket with white panels across it and black tracksuit trousers.
The second is described as looking around 13 years old, white, chubby, with a black jacket, blue hoodie and grey jogging trousers.
Police are still working to get a description of the third suspect. Enquiries are still being made at the scene and the incident is being treated as arson.
Anyone with information should call 101, select option 1 to speak to the Force Control Room and quote incident number 459 of today.
Video: Rebecca Howitt