Pupils from Dilton Marsh CofE Primary School have been presented with the Environmental Youth Award by the Duchess of Cornwall.
The awards recognise the efforts of young people in promoting environmental and conservation issues.
The scheme is open to schools and youth groups from Bristol, Dorset, Somerset and Wiltshire.
Headteacher Judith Finney said: “This is not about a single project – we have done so much over 15 years and we are going to continue to develop.
“The Duchess was absolutely charming and a very lovely lady.”
The school had already won the award for Wiltshire, but won the overall prize when the Duchess opened a sealed envelope on Sunday at The Bath and West Show.
They received a bronze trophy of a hawk and £500 in prize money to spend on environmental development.
The school has run a number of environmental projects including a First World War memorial garden and an insect garden, and it now plans to plant a woodland area.