The introduction of zero-tolerance behaviour policies leads to disproportionate numbers of black and ethnic minority students being excluded from school, research has uncovered.
Black teenagers are nine times more likely to be recommended for suspension under a zero-tolerance approach to behaviour than their white classmates, a study by Harvard University in the US suggests.
The racial disparities become even more marked when looking at the impact of the approach on teenagers with special needs, according to Stephen Hoffman from the Harvard Graduate School of Education.
Mr Hoffman spent three years exploring the effect of the discipline policy in middle and high schools in the US, but warned that the patterns he observed were likely to be replicated in other countries showing zero tolerance towards behaviour.
"The faculties of schools are typically composed of people who succeeded in school," he said. "But the population of society is large and quite diverse. If you put the hammer down hard on kids, you're going to drive them out of education.
"There are a lot of situations when they're going to get into trouble - or possibly killed - when they should be in school."
The findings come as tougher discipline regimes are being promoted in the US and the UK. Many of the most high-profile charter schools in the US and academies in England subscribe to a "no excuses" culture in which even minor infractions of behaviour policy are punished.
Ofsted, England's school inspectorate, has also backed "zero-tolerance" and "non-negotiable" behaviour codes. Sir Michael Wilshaw, the chief inspector, is well known for his strict regime when he ran the highly successful Mossbourne Community Academy in East London.
Mr Hoffman's research looked at the impact of the introduction of zero tolerance in one US school district of 37 schools.
Under the new policy, offences that had previously merited in-school punishments, such as detentions, required a five-day suspension. In addition, offending students were referred to the superintendent of schools for possible permanent exclusion.
"It takes the discretion away from principals," Mr Hoffman said. "In every case, you must impose the same consequences, regardless of particular circumstances."
The aim of the new policy was to reduce the number of temporary and permanent exclusions. In fact, it had the opposite effect. In the year before the zero-tolerance policy was introduced, 164 secondary students were suspended from school for five or more days. The following year, 241 teenagers were suspended for at least five days.
Then Mr Hoffman considered the impact of the new policy on black teenagers. The number of black students recommended for permanent exclusion more than doubled, from 2.1 per cent of students to 4.6 per cent in the years before and after the introduction of zero tolerance.
By contrast, the proportion of white students at risk of permanent exclusion rose from 0.3 per cent to 0.5 per cent. So, although less than a quarter of the students in the district were black, they accounted for three-quarters of the increased number of students being excluded under the new policy.
The racial disparities became even clearer when Mr Hoffman looked at the impact of the new policy on teenagers with special needs. More than half of black boys with special needs were suspended under the new regulations, compared with one in five white boys with special needs.
Phil Beadle, a teacher and educational consultant in England, criticised zero tolerance, saying it was the opposite of ensuring that the punishment fits the crime. "Rather than zero tolerance, there should be massive amounts of tolerance," he said.
"There's a radical difference between being extremely strict on behaviour - understanding that, without decent behaviour, decent learning can't happen - and having a draconian policy in which the punishment must exceed the crime's just deserts."
Mike Griffiths, head of Northampton School for Boys in England, also warned against the idea that zero-tolerance strategies would improve behaviour.
"It's a simplistic solution to a complex series of issues," he said. "In my school, we keep a very high standard of behaviour. But we do it through persuasion and discussion, rather than by using some metaphorical big stick if you step out of line."
Professor David Gillborn, director of the University of Birmingham's Centre for Research in Race and Education in England, said that the research backed up previous studies. "White teachers tend to discipline black students more quickly than they do their white peers involved in the same behaviour," he said. "And the punishments tend to be harsher.
"It reflects wider stereotypes. White people tend to view black people as more likely to cause trouble than to be in the gifted-and-talented set. These findings point to a need for much more sophistication when dealing with issues around exclusion."