The initiative has been introduced by the head teacher, Peter Walker. He plans to test pupils each week, with names selected by computer. The school is the first in the state sector to take up Tony Blair’s call in February for random testing to be available where heads wanted it.

Drugs charities and civil liberties campaigners said that the scheme could label children unfairly and lead to an increase in truancy by pupils anxious to avoid testing.

The swabs were administered by non-teaching staff at the school and sent to a laboratory for analysis of drug traces, including cannabis, Ecstasy and cocaine. Results are expected within three days.

“It’s taken a long time but we are finally getting started,” Mr Walker said. “This school doesn’t have a worse drugs problem than anywhere else. In fact, it is probably better because we are proactive.

“None of the pupils will be forced to take a test against their wishes. If they refuse, we will call the parents in to the school and discuss it.”

Pupils who test positive will not be expelled unless they are found to have been dealing in drugs. Mr Walker said that the aim was to support children with problems rather than punish them. He said that the tests would have a deterrent effect and would help pupils to refuse drugs if offered them.

Recent research found that almost a third of those aged between 11 and 15 admitted trying illegal substances.

Mr Walker sought permission for the drugs tests from parents of all 960 pupils at the school. Of 701 replies so far, 85 per cent supported the scheme.

Parents will be told of their children’s results and Mr Walker plans to publish a regular summary of the test findings.

Martin Barnes, the chief executive of the charity DrugScope, said that there was no proof that random testing was a deterrent, and Barry Hugill, a spokesman for the civil rights group Liberty, said: “We’re not at all sure what this is going to achieve apart from turning perfectly innocent children into supposed drug abusers.”

David Davis, the Shadow Home Secretary, said that a future Conservative government would “support, encourage and accelerate” random drug testing in schools.

The Department for Education and Skills has urged head teachers to exercise extreme caution over methods that might lead to “labelling” and be damaging to pupils.

Many independent schools have operated a form of random testing for nearly a decade.

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