Garden furniture was set on fire at Sir William Romney's school in Tetbury at the weekend.
Firefighters arrived on the scene shortly after 6.15pm on Saturday after being alerted by a member of the public.
A video posted on social media showed a bench next to the school well ablaze, but the fire was quickly extinguished.
The firefighters left the scene an hour later.
A spokesperson for Gloucestershire Fire and Rescue Service said: “We were called by a member of the public to reports of a fire at Sir William Romney School in Tetbury.
"One crew from Tetbury attended and used one hose reel to extinguish a fire affecting a bench and pergola."
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