Random drug testing in schools could give children a way of resisting peer pressure to experiment, according to an anti-drugs campaigner.
Pupils in the US have welcomed drug tests, says Peter Stoker
Peter Stoker, head of the National Drug Prevention Alliance, said the policy had helped pupils stand up to pushers when it was introduced in the US.
Supporters say some US schools halved drug use after bringing in the tests.
Mr Stoker told BBC Radio 5: "A lot of the pupils feel quite good about having a system like this in place.
"It gives them an excuse to say no when someone is trying to push them into using."
But he warned that head teachers must use the tests as a way of getting help to pupils who were dabbling in drugs, rather than punishment.
"Schools have to have a range of responses, which could be anything from a stiff talking to in the manner of a caution, through to referring pupils on to external counsellors or even treatment.
"Most important is that those consequences are not just punitive.
"Testing mustn't be used as a thin excuse to sling somebody out that a teacher doesn't like the look of.
"It has got to be related to positive intervention."
More than 1,000 US schools are now believed to have introduced testing, although a British teachers' leader warned the tests may be impossible in this country.
Jean Gemmell, general secretary of the Professional Association of Teachers, said: "My first reaction was to be fairly horrified, mostly because I cannot quite see how on earth it is going to work.
"Litigation is rife when teachers are deemed to have done anything intrusive that parents or young people are not happy with.
"All of these things are devised for good motives but the practicalities make mincemeat of those intentions."
At Hunterdon Central Regional High School in Flemington, New Jersey, pupils are selected randomly by a computer in the school's medical centre and then called out of classes to have their saliva tested.
It is reported the scheme has cut drug use by at least half in most year groups, after parents lost a court battle to stop testing when it was first introduced by the school more than three years ago.
Many US schools, including Hunterdon, ban children who test positive for drugs from joining in extra-curricular activities like cheerleading, chess and football.
A study by researchers at Oregon Health and Science University found that members of sports teams at schools with random drug testing were about four times less likely to take drugs than those at schools that did not carry out the tests.
In his recent State of the Union address President George Bush said testing had helped cut the number of classroom drug users by 400,000.
He told Congress: "The aim here is not to punish children but to send them this message: 'We love you and we don't want to lose you.'"