Two head teachers - one a bereaved mother - have warned of the dangers of drugs, saying privileged backgrounds are no protection against them.
Anthony Seldon, master of Wellington College in Berkshire, told a public schools' conference drugs were "too sinister" to tolerate.
He said children from rich and poor families alike suffered mental health problems as a result of drug-taking.
Elizabeth Burton-Phillips told how her two sons became addicted to drugs.
One son, Nick, committed suicide in 2004, after becoming addicted to heroin.
Mrs Burton-Phillips, head of Godstowe Preparatory School, High Wycombe, now campaigns against drugs.
She told the conference there was a culture of "denial" in top public schools over illegal drugs.
She said affluent pupils in public schools were prime targets for sophisticated "grooming" by drug dealers.
" The media and the government need to be far, far tougher and unequivocal"
Dr Seldon said attitudes to drugs were "too lenient".
He recommended schools adopt his own tough policy, which was to say there was "no second chance on drugs".
Dr Seldon was speaking ahead of a debate on mental health at the annual meeting of the Headmasters' and Headmistresses' Conference of leading independent schools in Bournemouth.
"I have a policy of 'no second chance' on drugs," he said.
"They are so evil, massively evil - even cannabis."
Having such a tough policy meant he had not been required to exclude any pupils from his school, he said.
"If you have a very clear, sharp policy then everyone knows where you stand."
'Lost his mind'
Dr Seldon said: "Government has toughened up but the media and the government need to be far, far tougher and unequivocal: no illegal drugs, no recreational drugs are acceptable in any form.
"I heard the other day about an adult who smoked a joint - his first joint - and he lost his mind for six months.
"You can just be unlucky. You can have this predisposition which can tip you into psychotic disorder and malfunction which can be cataclysmic and from which some people can never recover their baseline sanity."
Earlier this year, the head of Scotland's drug enforcement agency Graeme Pearson called for a radical shake-up in drugs education in schools, saying the present system was not working.
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