Drug use among children is being concealed by a "zero tolerance" attitude among schools, Home Office research has indicated.

Education department guidelines encourage children "to conceal rather than deal with their drug use", the study of 300 young offenders concluded.

The research claims that school powers to automatically expel pupils caught dealing in drugs result in them being excluded from society and pushed towards crime.

"They are not necessarily those who use drugs most and not the only users in school," the report said.

In a report commissioned by home secretary David Blunkett, the Essex University researchers also rejected claims of a link between "recreational" drugs and harder substances.

They said that their sample of offenders were not being drawn into use of hard drugs, even though they were regularly using lighter stimulants such as ecstasy, amphetamines, cocaine, LSD and cannabis.

"There was no evidence of a progression towards heroin or crack cocaine use or dependence despite the diverse drug use amongst the group," the report said.

"The shift towards the use of heroin and/or cocaine and/or drug injection observed in the 1980s amongst delinquents was not evident."

It found that "alcohol, tobacco and cannabis were more strongly related to offending than were other drugs".