The Schools Health Education Unit this week published figures indicating that more than 10 times as many teenagers now use cannabis than did two decades ago.
However, David Regis, head of research at the Exeter-based unit, said schools deserved credit for their effective work de-terring pupils from trying illegal drugs.
The proportion of 14 to 15-year-olds who say they smoke cannabis rose from 2 per cent in 1987 to just under 30 per cent in 1998.
Since then the number taking the drug has barely changed.
But since since 1998 the proportion being offered the drug has risen substantially, from less than 30 per cent to 45 per cent last year.
Dr Regis said the figures indicated that young people were being better prepared to refuse the drug.
"While it's far from good news that so many young people are exposed to cannabis, it seems that many of them are willing and able to refuse offers of cannabis and other drugs," he said.
"We know many schools go beyond the nuts and bolts of drug information to look at social skills like refusing unwelcome offers and resisting peer pressure. We are able to counter the headlines about the 'doped-up generation'."
Another positive discovery by the unit, which has surveyed more than 400,000 pupils since 1987, was that the proportion of young people taking more dangerous drugs has declined.
Use of ecstasy, amphetamines, solvents and hallucinogens such as LSD by teenagers have all fallen since the mid-1990s.
Pupils also seem more knowledgeable about the dangers of drugs, with many more recognising the risks of amphetamines in last year's survey than in previous years.
The Drugs Education Forum has written to education minister Lord Adonis in light of the report, urging him to make learning about drugs a statutory part of personal social and health education.
Details from Trends: 1987-2004 Young People and Illegal Drugs are at www.sheu.org.uk