Changes in the law on cannabis have sparked an epidemic in use of the drug among Scottish youngsters, headteachers have warned.
A study found a third of 15-year-olds in Scotland have used cannabis
Smoking a joint is as common as "a fag behind the bike sheds" was a generation ago, the Headteachers' Association of Scotland (HAS) said.
Spokesman Alex Easton said many young people were confused by conflicting signals from police and the government.
He said that many believed cannabis to be harmless and in some cases legal.
The warning follows the trial of Luke Mitchell, the murderer of Jodi Jones, which heard he smoked large amounts of the drug.
Mitchell, who is facing life after being found guilty of murdering his 14-year-old girlfriend in Dalkeith, Midlothian, in June 2003, once bragged of smoking 600 joints a week.
Mr Easton, who is rector of Falkirk High School, said: "Cannabis may not be as bad as heroin or cocaine but it's far from harmless or safe - recent medical research has shown it can cause psychosis.
"But many young people are unaware of the potential side-effects and out with schools it's now the equivalent of what a fag behind the bike sheds was 35 years ago."
A recent study found more than a third of 15-year-olds in Scotland had used cannabis and almost one in eight had tried drugs such as cocaine or ecstasy.
Mr Easton said the Scottish Executive's Know the Score campaign should turn to clarifying the law and health issues of cannabis.
"Young people don't know the difference between decriminalisation and legalisation and are confused about the conflicting signals they get from the government and different police forces.
"Many see cannabis as a safe recreational drug. When was the last time you saw a TV campaign about cannabis? A health and education advert would be really beneficial in youngsters to help them make appropriate choices."
The Scottish Socialist Party, which wants to see cannabis legalised, backed the calls for more clarification.
The party's drugs spokesman, Kevin Williamson, said: "There ought to be an honest informative initiative on what the side effects if cannabis use can be.
"But we should also stop blaming cannabis or looking for reasons to blame what happened in that Jodi Jones case.
"Instead of blaming youth culture we should accept young people are under more stress these days and a lot use cannabis just to chill out like other people."