Peter Smith, general secretary of the Association of Teachers and Lecturers, said drugs education should be a central part of training for all new and serving staff.
He warned that lessons teaching children about drugs should have clear moral content. "The sort of drug education which says `this is what drugs do, this is the upside and the downside' - that sort of open ended lesson in my judgement is irresponsible," he said.
Delegates at the association's conference in Bournemouth voted to press ministers for an expansion in drugs training after teachers said they needed help to become more streetwise about the subject.
John Beattie, the union's vice-president, said: "Youngsters think that they know it all, and think that teachers know nothing while we believe they do not know enough."
But some teachers warned that anti-drugs sessions at school could actually encourage young people to experiment.
David Britton, a teacher at the Skinners' Company's School for Girls in Hackney, east London, said he was approached by a girl after one drugs awareness lesson. "She said `these people are actually making us feel that we want to try drugs, which we would never dream of trying'. Where you have a situation like this it poses questions about the job should be approached."
Other delegates claimed increasing problems with drink and solvent abuse. David Lutwyche, a master at Lancing College boarding school in West Sussex, said: "On one occasion one lad landed a right hook on my wife before passing out after consuming a bottle of vodka and several cans of lager. That lad is spending this Easter holiday at a private clinic trying to dry himself out to take his A-levels."
Shirley Blackman, a member of the association's executive, said "I have friends' children and a child of my own who have experimented with drugs. Those children have been handled by people who care and are interested in their education and they are still at school. They have not been turned out, they have turned around and they have turned away from it."
Teachers said children were becoming involved with drugs at an increasingly early age.
Mr Smith said children as young as eight or nine were involved. He said: "Drugs in this country are endemic. The evidence at the moment is very impressionistic, but certainly our primary school teachers are very conscious of the fact that they have had no training, that there is very little opportunity for drugs education to take place, and there is a problem."
A union poll published yesterday found that one-third of secondary school pupils feared that their teachers did not know enough about drugs.
The MORI poll of nearly 4,000 children found that 22 per cent thought teachers were not very aware of drugs. Another 11 per cent thought staff were not at all aware of drug abuse and 44 per cent of all 14- to 15-year- olds thought teachers lacked knowledge about drugs.
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