Almost 30 per cent of independent-school pupils have experimented with drugs by the time they reach 14 - and more than 10 per cent claim to be regular users, according to unpublished research.
However, many independent-school headteachers are largely unaware of the scale of drug-taking and drinking among pupils. Most rate parents' marital break-up as a greater threat to their pupils' well-being, according to a study carried out for the Headmasters' and Headmistresses' Conference (HMC). Sex fails to register as a concern.
Privately-educated pupils appear marginally less at risk from illegal substances than their state-school counterparts. Nationwide figures suggest that one in three 14-year-olds and two in five 16-year-olds have tried drugs at least once.
The HMC is due to publish the report in the new year, but chairman Patrick Tobin disclosed some of its findings to more than 200 parents at a closed meeting at the House of Commons earlier this term. He declined to release the full findings early, but said they would help some headteachers to "focus their minds".
"We want to bury the lie that the HMC is always on the back foot. We are saying these are problems in society and every school has got to do something about it. It is incumbent on every school to have its own drugs policy, " he said.
He emphasised that the majority of pupils were steering clear of drugs altogether or only briefly caving in to peer pressure to experiment. Both drink and drug abuse were also societal problems, he added.
The research canvassed the views of HMC headteachers on drugs and alcohol, and questioned 2,400 teenage pupils at 20 schools about their actual use.
The survey showed pupils' contact with both drugs and drink was much higher than that estimated by their headteachers. When it came to drink, two-thirds of boys and three-fifths of girls had been drinking in the previous week.
"On alcohol, I don't see it as being a specifically under-age problem, if you look at young professionals' or parents' drinking. Pupils are not getting much of a good example from my generation. With drink-driving, their generation is far more responsible than mine," said Mr Tobin.
The HMC is recommending that its members review pastoral care systems, produce whole-school drugs and alcohol policies, and include drugs and alcohol education in properly-funded personal and social education programmes.