Introducing random drugs tests in schools is an "extreme measure" which may drive abuse underground, a drugs information charity warns.
DrugScope says the emphasis must be on education
The criticism from DrugScope follows news that secondary schools in Kent will be carrying out the tests.
But DrugScope fears the move could undermine an open relationship between pupils and their teachers which can allow drug misuse to be debated freely.
The charity has instead called for a focus on drugs education in schools.
The decision to introduce the tests in Kent's schools follows a scheme at Abbey School, in Faversham, where pupils were randomly selected and tested by taking mouth swabs.
The school said it was one factor which led to record GCSE results in 2005.
Now Kent County Council is asking other schools if they wish to join a pilot scheme for the tests.
"We're quite concerned Kent is considering rolling this out in other schools," said DrugScope spokeswoman Petra Maxwell.
"There is absolutely no evidence to suggest random drugs testing have lowered drugs use."
Ms Maxwell said the "intrusive programme" was inappropriate in the context of a school.
Testing was said to help Abbey School get its best GCSE results
"Schools may be a first point of call for pupils with problems," she said.
"Open debates, allowing pupils to openly discuss the issues, are unlikely to be compatible with a school that has random drugs testing.
"These tests are drastic measures and could have a negative impact on the ethos of a school and relationships within the school.
"And teachers are professionals, they're the experts on young people, they don't need a drugs test to tell them a pupil has a problem."
Ms Maxwell also expressed concern about children who were outside the school system, either through exclusion or truancy, saying they were more likely to be taking drugs, especially class A drugs.
She said education was the key to preventing abuse among youngsters, particularly because it took into account legal substances, notably alcohol and tobacco.
"We'd prefer to see the focus on drugs education."
Head teachers' decision
The teachers' union, the NASUWT, said it supported the concept of random drug testing.
"Such strategies have the potential to both deter and detect," said general secretary Chris Keates.
"Whilst the pilot in Kent is welcome, it is disappointing it is to be confined to only one authority.
"I hope that others will press the Department for Education and Skills to include them so that the effectiveness of the strategy can be tested in a range of authorities."
A spokesman for the DfES said the decision to introduce drug testing in schools was a local one.
"As we have made clear throughout, the decision to adopt this approach will be one for heads in consultation with parents.
"This is about providing schools who want to look at this approach with the best possible information and guidance rather than forcing them to do it."