A gazebo and outdoor area used by children with special educational needs has been set on fire.
Firefighters from Thornbury, Patchway and Yate tackled the blaze at the Castle School on Christmas Eve.
A parent has already set up a crowdfunding page to help rebuild the wooden gazebo in the new year.
Responding to what had happened they said the school needs a boost and that the community rallying together to raise funds will "show the cowards who are trying to bring the school down that they will not win."
Avon Fire and Rescue believe the fire was started deliberately just before 9.45pm on 24 December. The police and fire service are now working together to investigate the crime.
It comes just a week after officers in Thornbury announced they had arrested three teenagers on suspicion of arson relating to a different fire at the school's sixth form.
Police are now asking parents to check their child or children's mobile phone for photo or video evidence of the fires.
"If anyone is sent/has photos or footage from social media relating to these fires can the posts be screen shot and shown/given to police. This includes parents checking their childs accounts and assisting police."