A growing number of schools are introducing drug testing in an effort to deter pupils from experimenting with illegal substances.
A spokesman for Biogenerics, a Scottish company which offers a range of "schoolyard drug detection devices", claims that more than 250 British schools have used the products.
Concerned parents are also being offered "Catch and Cure home drug test kits", in which samples of urine or strands of hair can be collected. Urine collection pots which indicate the presence of drugs are sent for laboratory analysis which can detect traces of up to 32 drugs.
The company claims to be offering "a practical and powerful deterrent against drug trading and possession in schools". Its publicity states: "Biogenerics offers parents a practical, pro-active approach to drugs, and puts the parent back in control. If a child is using drugs then early detection (by whatever passive means) is justified."
Biogenerics began selling the kits two years ago but now supplies them free to schools and parents after drug-advice agencies accused the company of profiteering.
However, health professionals are still highly critical of its tactics. John Balding, director of the Schools Health Education Unit at Exeter University, who has conducted a survey of drug use among 50,000 children, said the company's methods were not pro-active, but destructive, and could lead to a breakdown in trust between parent and child.
Dr Manjit Ruprah, head of the drugs abuse department at the National Poisons Unit, said: "I'm totally against it. We would never recommend this kind of do it-yourself drug testing." Proper testing required a guaranteed chain of custody which could not be achieved by sending samples through the post, she added.
Sally Taylorson of the drugs advice charity Release said: "This is quite outrageous. It is very, very sneaky - some of these tests can be done without the child's knowledge or permission." Communication between parent and child was the key to overcoming problems of drug abuse.
Drugs testing has been introduced with parents' consent at several leading public schools. Eton, Millfield, Sevenoaks, Wellington and Westminster schools are among those to have expelled or suspended pupils this year for taking drugs.
Dr Roger Harrington of the Medical Officers of Schools Association, which represents doctors at independent and maintained schools, could not confirm Biogenerics' figures, but said that around 40 MOSA member schools tested pupils for drugs. Dr Harrington, who is medical officer at Stowe school in Buckinghamshire, said there were ethical problems with random testing .
Also, any break in the chain of custody of a sample would make a positive test legally indefensible and lawyers representing children who had been expelled in these circumstances were preparing a legal challenge to drug testing in schools, he added.
The issue was being debated in Dublin last week at the Headmasters' Conference, the annual meeting of independent schools, and at a one-day seminar in London organised by Release.
David Wallace, a spokesman for Biogenerics, defended his company's activities. "Parents have a right to know if their child is or is not on drugs. Even if the result is positive, that can be a relief.
"Initially we were naive in our approach and we stopped charging for the kits because we didn't think it was appropriate as a lot of the parents were on income support.
"The drug agencies are dead against pro-active detection. They have done nothing but bombard us from day one. You would think we were dealing drugs and not trying to stop them." He said that re-testing had shown that the kits had a 90 per cent success rate in dissuading children from taking drugs.