More school pupils could face tests for drugs as part of studies to assess whether such moves affect behaviour, attendance and academic achievement.
The government's advisers want drug testing to be abandoned
A government-backed pilot is running in Kent and other schools in England are being invited to take part in trials.
Head teachers who have already tried random, voluntary drug tests say they have had a positive effect.
But the government's own drug advisory panel has raised concerns about the ethical and legal issues involved.
The Advisory Council on the Misuse of Drugs, which also believes there has been a lack of evidence that testing is effective, has recommended that random tests be abandoned.
A Department for Education spokesperson said the findings from the schools involved in the Kent pilot were yet to be published.
The current pilot and the other trials, which are set to run for six months from January, are being organised by government consultant Peter Walker, a former Kent head teacher.
Charity DrugScope has also previously warned that testing in schools is an "extreme measure".
That warning, in May, followed news that secondary schools in Kent would be carrying out tests as part of the pilot scheme.
DrugScope says there is no evidence to suggest such testing has lowered drugs use and such "intrusive programmes" are inappropriate in the context of a school.
But some head teachers who have tried random, voluntary testing say it gives children the power to say no to drugs outside school and improves attendance rates and behaviour by making pupils feel safer.
Colne Community School in Essex has been running trials of random drug tests for more than a year.
No pupils have so far tested positive, but the school says that is not the point.
Head teacher Terry Creisson said: "We wanted to give young people an excuse to say no.
"Having drugs education programmes in schools is all well and good but having the drug testing programme means that they can turn round to their friends and say 'I can't take drugs because I could be tested next week'."
Research which suggests about one in four children have tried drugs by the age of 15.
Under the voluntary procedure, a saliva sample supplied by a pupil can be tested for traces of cannabis, amphetamines, morphine and cocaine.
Results can be achieved in about 20 minutes, and the child - or their parents - can refuse a test at any time.
Each testing kit costs about £13.
Mr Walker said the tests were easy to do and produced good results.
"It doesn't cause any harm whatsoever, a child still has a right to say no, indeed so does a parent, it doesn't disrupt the running of the school, it's relatively cheap to do and brings great benefits like improved performances," he said.
But Vivienne Edwards from the Advisory Council on the Misuse of Drugs said: "Our recommendation is that drug testing shouldn't be used in schools because of the potential impact on the pupil-teacher relationship and because of the complex technical and organisational and ethical issues that surround it."
She added there was so far no evidence that such testing reduced drug taking among young people.