Rising exclusion rates among pupils found with drugs may be undermining schools' key role in tackling substance abuse.
Schools have been told they are central to a long-term assault on the UK's drugs problem, through anti-drug programmes and personal, social and health education.
But drugs educators say pupils excluded for relatively minor offences by schools trying to appear tough on drugs are being kicked out of the system that may help them address their behaviour, and putting them in increased danger of addiction.
The warning comes as local authorities in Kent and Leicestershire launch initiatives to keep young offenders in school.
In Kent, heads in a growing number of schools have agreed not to exclude young people found with drugs if they attend an intensive six-week course run by the youth service, police, health, education and social services.
Mike Trace, deputy UK drugs czar, said: "Generally, exclusion for possession of drugs is too harsh a punishment and creates more problems "Those at risk say the quickest way to accelerate a drug problem is to kick them out of the mainstream. For school-age children that means kicking them out of school."
No records are kept of the reasons pupils are excluded, though Mr Trace and his boss, drugs czar Keith Hellawell, believe drug-related exclusions are rising. They have told the Department for Education and Employment to start keeping records from April, and cutting drugs-related exclusions is a target of the Tackling Drugs White Paper.
The important role of schools was highlighted by Mr Trace and by cabinet minister Ann Taylor at a conference of Yorkshire head- teachers last week.
But speaking to The TES, Mr Trace said many schools failed to distinguish between possessing drugs and behaviour caused by drugs. Half of all teenagers report having used an illegal substance at least once by the age of 16.
Kent County Council and Kent Constabulary hope to have their scheme, the Drug Support Programme, taken up by all schools in the county.
Some 85 young people aged 11-17 have already gone through the programme, which consists of six two-hour sessions at their local youth centre. Using group work and one-to-one sessions, they covered the law surrounding drugs, the youth justice system, health issues, and support agencies.
Most participants were caught in school in possession, or arrested out of school, though some were referred because they were thought to be at risk of drug use.
Bill Butler, of the youth and community service, said the biggest challenge was convincing schools the programme was not an easy option. It also meant persuading police and the Crown Prosecution Service to suspend cautions or prosecutions against offenders.