A new learning hub in the grounds of Darnley Primary School in Glasgow's South Side was completed only last Friday after months of planning and work.
But "devastated" staff and children returned to the school, near Pollok, to find the structure had been battered and broken.
The large gazebo – built as part of a £5000 council-funded outdoor project – is now a mass of splintered wood.
Teachers planned to use it to host outdoor learning projects with pupils, including children from the school's visually impaired unit.
They now face having to find more money to repair the structure.
Police are investigating the vandalism and a senior councillor hit out at the culprits branding it "disgraceful".
Head teacher Rona Bernstein told the Evening Times: "This is a devastating act of mindless vandalism.
"The children watched the gazebo being completed all last week and excitement was building as it neared completion.
"But then I had to explain to them that it had been badly damaged.
"The children felt like this was their property and they can't understand why someone would do this to them."
Principal teacher Lindsey McBride fought to secure funding for the project. This came from Glasgow City Council as part of the Outside Now learning scheme.
The first £1000 was spent on an outdoor play area and the other £4000 split between the gazebo and an open air seating area.
Eco charity, Glasgow Wood Recycling, worked throughout last week to build the gazebo.
Miss McBride said: "This is galling because the children didn't even get to use it once."
Councillor Stephen Curran, executive member for Education and Young People, said: "It's disgraceful there are people out there that would target a local primary school this way.
"I urge anyone with information to contact the police and hopefully catch whoever is responsible."
On seeing the damage, P4 pupil eight-year-old Shannon Fry said: "I really hope that we can get it fixed."
Abby King, nine, of P5, said: "I hope the police catch the vandals."
Anyone who can help trace the vandals should call 0141 535 5600.