Prince Harry has admitted using drugs and alcohol

In response, schools often rely on prevention as much as punishment, with automatic expulsion only likely to apply to pupils caught drug dealing.

And he says that attempts by state schools to use drug testing to identify users have not been successful.

There were concerns about the ethics of testing young people - and legal doubts over whether pupils could be forced to take tests.

Mr Carstairs said that there were also concerns that widespread testing would be too expensive for schools.

Discipline

Drug testing, which usually takes the form of a urine test, has been more widely used in independent schools.

But this has not been in the form of random testing, but as part of a disciplinary process imposed on pupils caught using drugs.

Independent schools sometimes require drug-using pupils to take tests as a condition of staying in the school, says Dick Davison of the Independent Schools Council.

But he says that, as with state schools, a majority of independent schools will not automatically expel pupils caught using drugs.

Both the state and independent sectors have to tackle a problem which is brought into school from the wider community - and much of their efforts go into drug education.

Drug prevention

By 2003, all state secondary schools will be required to have an anti-drugs strategy in place, with �14.5m in funding already promised for this year, rising to �17.5m next year.

And already, all secondary schools provide lessons in drug awareness and warn pupils about the dangers of drugs and alcohol.

But such lessons are up against a youth culture in which the use of alcohol and drugs is widespread.

A survey of over 30,000 secondary pupils early this year found that among 16 year olds, 24.7% of pupils were regular smokers, 40.8% drank alcohol regularly and 7.7% regularly used illegal drugs.

The survey, carried out by Adolescent Assessment Services, found that younger pupils were already experimenting.

Among 12 year olds, 6.3% were smokers, 8.3% drank alcohol and 0.8% used drugs.

And Mr Carstairs said that the union had received calls looking for advice from head teachers unsure how to discipline young pupils who had come to school with hangovers.