Teachers have expressed concern about Tony Blair's plan to allow random drugs tests on schoolchildren.
Head teachers will have the power to offer treatment to young users, exclude them or report offenders to police.
The head of the NASUWT teaching union said testing was a "last resort", which could alter the relationship between teacher and pupil.
Another teachers' leader said the proposals may lead to legal action by pupils against staff.
However a parents' group said it was "cautiously positive" about the plans, which will be introduced in the form of guidelines for schools in England next month.
Eamonn O'Kane, general secretary of NASUWT, said drug testing would be a "very big step" for any head teacher to take.
"It is effectively giving them police powers and I think a head teacher would want to think very, very carefully before exercising them," he said.
But he added: "No school can tolerate drugs and a head faced with such a situation may look
(at the proposed new powers) as a last resort."
"I certainly would not like to see individual teachers involved in such testing."
Jean Gemmell, general secretary of the Professional Association of Teachers, said: "I cannot see how it could work and, as a former head and someone who represents teachers, it is adding to their burden of social responsibility to the point that it becomes untenable.
"Litigation is rife when teachers are deemed to have done anything intrusive that parents or young people are not happy with."
Dr John Dunford, general secretary of the Secondary Heads Association, said the proposals unfairly placed the burden of preventing drug abuse on schools at a time when drugs education funding had been cut.
"I am concerned at the implication that the drugs problem is rooted in schools and that schools should solve it.
"It is ironic that the prime minister is proposing this at a time when the Department for Education is abolishing its special grant for drugs education."
David Hart, general secretary of the National Association of Head Teachers, backed the plan but warned Mr Blair it would only work with the support of head teachers.
He said: "It would give them another weapon in their fight against drugs being pushed or used in their schools."
In an interview with the News of the World, Mr Blair said the guidance on new powers for head teachers to test for drugs, including using sniffer dogs, will be issued next month.
The National Confederation of Parent Teacher Associations welcomed the prime minister's announcement, but said many questions still needed to be answered.
Spokeswoman Margaret Morrissey said: "We are cautiously positive about the proposals.
"There are a lot of questions parents will want answering, firstly if it is to be voluntary or enforced and also, what provisions are going to be there for support if a head does find a problem."
The BBC's political correspondent Guto Harri said the proposals were an attempt by the prime minister to focus on domestic issues ahead of the next election.
He said: "The government want us all to believe they really care about those bread and butter issues that most of us will be thinking about when we next vote.
"Interestingly, Tony Blair is saying to head teachers: 'you can do this', he is not saying to them they have to - a sign perhaps of just a little bit of humility."
Results from an ICM Research poll which appears in the News of the World on Sunday suggests 82% of parents and 66% of children support drug testing in schools.
But the Liberal Democrats and drugs groups have condemned the plans, while the Conservatives said it sent out a mixed message so soon after the downgrading of cannabis to a Class C drug.