Emergency services were called to the scene, on Queens Avenue, shortly after 9pm last night after football nets on the field were set on fire.
It is believed by police that the fire on the fields, which are owned by the school but accessible to the public, was deliberately started and they are now investigating the incident.
One fire engine from Dorchester attended the incident at 9.20pm, and fire fighters extinguished the fire using buckets of water.
A spokesman for Dorset Police said: “We were called to the scene following a report by Dorset Fire and Rescue Service.
“The fire has been classed as deliberate, with the football nets in the field set alight.
“No arrests have been made and nobody was injured, but we are now conducting inquiries into the incident and we are asking people who may have witnessed it to come forward.”
To contact Dorset Police regarding the incident, call 101 and quote the crime number 17:448