With the unfettered competition between schools, it is believed many are now worried about the damage to their image if their pupils are involved in drugs offences.

The cover-up charge was levelled by David Moore, the HM inspector nationally responsible for pupil behaviour, who has identified “a number of cases”.

“I don’t understand how governing bodies can act outside the framework of the law,” he told council education officers meeting in Warwick last weekend.

Schools have no legal obligation to call the police when drug offences or incidents are discovered on their premises, even though Department for Education and Employment guidelines recommend that the police are called.

The Association of Chief Education Officers has now called for a joint campaign with the police and social services directors to give governors clearer advice on tackling incidents of drugs offences.

The idea has been backed by Tony Butler, the chief constable of Gloucestershire, who is to put it to the Association of Chief Police Officers following a meeting last week with education officials.

Mr Butler, who speaks for ACPO on juvenile issues, told the Association of Chief Education Officers: “We need to know if your school is the trading post for Ecstasy or amphetamines.

“The police will not always take an enforcement view, but we need to work out good relationships. We can give each other intelligence to make more informed decisions about children.”

Concern about the real extent of school drug-taking was underlined by headteacher leaders this week, who identified a growing problem of drugs in schools with ever-younger children getting involved.

David Hart, general secretary of the National Association of Headteachers, said some heads wanted to keep drugs offences under wraps.

“It is indicative of the impossible situation that heads are being put into - they are caught between the need to inform the police and their concern to protect the reputation of the school.”

He believed, however, that the vast majority would inform the police and said: “The need to ensure that the police are called in should be the prime concern, to make sure that a clear message goes out to the rest of pupil population that they not only risk exclusion but police involvement.”

Just last week a primary school in the London borough of Barnet excluded a nine-year-old boy for alleged possession of cannabis.

But there is now a growing consensus that such well-publicised cases are merely the tip of an iceberg.

The Department for Education and Employment has no statistics for the number of pupils excluded for drugs offences although overall levels of exclusion have risen since 1990 to 11,000 annually.

Anecdotal evidence collected by the Office for Standards in Education - and published last month - suggests numbers excluded for drugs offences have remained stable. But no figure is given.

Chief education officers suspect that many schools simply encourage pupils involved in drugs to leave the school rather than exclude them and create a fuss.

Eric Wood, county education officer for Warwickshire, said that there was a huge “iceberg-like” problem of drugs in schools.

“It cuts across the academic and social divide and it is a massive issue for families.

“You have a youngster with everything going for him or her and then out of the blue there is the possibility of a problem and the whole family is in turmoil. ”

Independent schools decided more than 10 years ago to introduce testing for pupils suspected of drugs abuse and several, including Eton, Harrow, Marlborough and Rugby, now carry out tests.

All children in state schools - from key stage 1 upwards - have to learn about drugs as part of the national curriculum.