Den areas, a fire pit, outdoor seating and buildings were targeted in the area used by Warkworth Primary School.
Laura Ritson, headteacher, said: "Everyone is upset by the damage that has been caused to our Forest School.
"Our den areas, fire pit, outdoor seating and building areas have all been destroyed. There is fire damage to the trees in the area and broken glass everywhere, it’s really sad to see.
"The site has been a real labour of love for our pupils, staff and parents who have built it up over the past eight years and have wonderful memories of the time they spent there.
“The Forest School is somewhere that all of our pupils go to learn practical skills, such as how to look after the environment, how to build from natural materials and how to cook on an open fire, but it’s also a huge part of our mental health and wellbeing work, as pupils get to be hands on with nature and have time out to be children, learning lifelong skills along the way.”
The incident has been reported to Northumbria Police.
Volunteers have helped to tidy up the mess and efforts will now turn to funding a restoration project.
Mrs Ritson added: “We’ve been overwhelmed by the response from the local community to help us with the clean-up operation on the site.
"Our school motto is to #StriveTogetherAsOne and with the support of our pupils, parents and the community, we hope that from the ashes, we can rebuild something even better for our children to enjoy as we head into the summer months.”
Alan Hardie, chief executive at Northumberland Church of England Academy Trust, added: "Opportunities to take part in outdoor activities, such as Forest School, are vital for the wellbeing of our pupils, so it is devastating that it was deliberately destroyed.
"However, Warkworth is a school at the heart of its community, and the strength of the community spirit is demonstrated by the number of people who have already volunteered to help us rebuild the Forest School.”
It is located around a mile away from the main school site. The damage is believed to have taken place through the most recent lockdown.