Trallwn primary, in Swansea, recycled a netball post to make a tree and filled old milk cartons with the children's pledges to do their bit to save the world.
As a result, the school beat 200 other entries to green charity Global Action Plan's first prize of a stained glass window. Plus 16 tonnes of water have been saved and Trallwn looks set to shave thousands of pounds off its electricity bill.
The project, to mark World Environment Day, also covered key skills such as design, numeracy and writing for the Year 4 pupils.
"I wanted to take part because the first prize was fabulous," admits teacher Sian Hopkins. "And this was a real design technology project that the children themselves could do."
The children started before Easter, with small prototype trees. They then found the netball post and covered it with industrial waste from a nearby enterprise zone to make the bark and foliage.
A doll was hung from the branches in a bubble wrap swing to symbolise the earth hanging in the atmosphere. But, like Topsy, the tree just kept growing.
"Every child was given a milk carton to put their promises into. They were hung on the tree as leaves. Then a lot of parents asked if they could do that too," said Miss Hopkins.
"We got 1,300 promises altogether and it was a nightmare, the whole thing became so tall it was almost too much for the tree."
The school has now pledged to carry out an audit to cut water use and the children have promised to treat home like school - switching off lights and saving water.
"We're delighted," said Eric Starkey, Trallwn's headteacher. "We're in a deprived area, half of our pupils have school meals and there is 25 per cent year-on-year migration.
"But we believe in giving the children a rich and varied experience, and this shows what you can do despite those problems."
As a result of World Environment Day promises made by UK schools, more than 100 tonnes of water will be saved, 1,040 batteries will be kept out of landfill, 1,700 children pledged to use both sides of paper, and 1,610 children promised not to waste power - saving schools an average of pound;375 each.