One in three young teenagers have used cannabis, some on a daily basis, according to new research. The study appears to confirm growing fears among teachers that joints have become the 21st-century equivalent of smoking behind the bike sheds.

School heads and administrators are so worried about the drug that they will gather this week at the first conference of its kind to address the issue.

Researchers found almost all those using cannabis on a daily basis were also smoking and drinking, more than six out of 10 had used ecstasy, nearly a third had tried cocaine, and one in 10 had tried heroin. The findings are based on a long-term study by Queen's University, Belfast, of 4,000 pupils aged 14 and 15 and will be published this week in the journal Drug and Alcohol Dependence.

Dr Anthony Seldon, master of Wellington College, Berkshire, and organiser of the conference on Friday at Wellington College, said: "I don't think there is a secondary school in Britain that doesn't have a problem with drugs."

He is gathering together some of Britain's top teachers, as well as senior police officers and experts from drugs and mental health charities, in an attempt to find a way forward.

One of the speakers, Bernard Hogan-Howe, Chief Constable of Merseyside, will be calling for a review of "out-of-date assumptions" about cannabis, warning: "The legacy of people taking this increased-strength drug today could be felt for generations to come."

Debra Bell, a parent whose 19-year-old son has just entered treatment after five years of struggling with cannabis abuse, will be calling for a health education campaign in schools. "Thousands of families across the UK are in crisis," she said.

All this comes as a ferocious debate continues over the mental health risks associated with skunk - a potent new form of the drug far stronger than traditional cannabis - first reported in this newspaper last month.