Cassop has a 50kw turbine in its school field, enough to power the whole school.
"We need a wind speed of five metres per second to generate power," says Mr McManners. "To model that for the children we ran races to work out how fast they could run in metres per second."
At 15mps, a very windy day, the power is enough for all the school's needs, with surplus energy fed back into the national grid.
The turbine was opened by Tony Blair in 1999, and is only one of many innovations at Cassop - possibly the greenest school in the country.
The school serves two former mining villages. Until 1983 the landscape was dominated by massive pit heaps and a colliery. The area once again produces its own power, but the turbine produces no pollution.
Cassop is heated by a boiler using recycled wood pellets. The pulverised wood comes from Durham's biggest landfill site, conveniently sited next door to the school.
The school has a garden maintained by the children. An early development was the planting of 100 trees, one for each family with a child at the school.
"Each family has a tree, and they know which is theirs," says Mr McManners.
There's also an annual walk around the village, plans for solar panels, an energy workshop and a healthy eating policy. The tuck shop sells fruit portions, and is run by the children.
Children have been involved in environmental projects and each year at lambing time Year 1 visits Mr McManners's smallholding, to see his collection of Swaledale and blue-faced Leicester sheep.
"I've been teaching for 30-odd years, and I just know that when we have work of the highest quality, when children really function at their best, is when they have first-hand experience - and environmental material is ideal for that.
"I think the loss of Eco-Schools funding is a great shame. Eco-Schools offers people a practical way to make progress."
The Office for Standards in Education has commented on the educational gains for schools who opt for sustainability. A report last year found that education for sustainable development (ESD) worked best when pupils were encouraged to initiate changes. Inspectors noted that it had been linked to drops in vandalism.
Mr McManners believes ecology education must be practical. "You have to enthuse the children about the environment and then they see a purpose in protecting it."