Three teenagers started a fire after trespassing on the grounds of a school in Wotton under Edge.
The incident occurred shortly after 1.30pm on Sunday at Blue Coat Primary School on Symn Lane.
Three teenagers between 12 and 15 years old lit a fire on the school grounds.
Police are now appealing for information.
A neighbourhood alert issued by PCSO Hazel Pearson said: "I am disappointed this has happened,
"Recently teenagers have been trespassing on the school grounds leaving litter, now fires are being lit, this is not just an arson but endangering the children at the school with dangerous litter."
If you have any information concerning this incident call 101 quoting incident number 198 or March 7.
Alternatively, you can give information anonymously by contacting Crimestoppers on 0800 555 111.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here