Two organisations declared their interest: Devon Direct Services, an offshoot of the county council, and Channings Wood Prison. The prison offered to do the work for Pounds 800: Devon Direct wanted Pounds 771. The prison won.

But Derek Green, Unison assistant branch secretary, is concerned that the trend for prisons to tender for such contracts will spread nationwide under pressure from the Home Office for the prison system to pay its own way, and is urging the union to take strong action now.

But what he says alarms him most is the potential danger to pupils - even though the Channings Wood tender specifically arranged for the work to be done outside school hours.

“You know and I know children don’t leave the school grounds at 3 or 3. 30. If there are good sports facilities in the school they climb over the fence and use them. My big worry, even if the prisoners are supervised and trusted, is that they will either abscond or do something that we don’t want to happen to the children in the area. There is the full range of criminals in the prison - rapists, murderers, drink-drivers.”

Head Len Peach dismisses the union criticisms. He says a major reason for choosing the tender was the intention to work outside school hours, because prison staff will do the bulk of the work. Prisoners - who will be supervised - will only be involved when more extensive work needs to be done.

The staff are not warders of the category C open prison, but run its extensive agricultural operations.

Roger Brandon, Channings Wood governor, said that although prisons were being encouraged to tender for contracts, prisoners would not come into contact with schoolchildren.