The best-laid plans of ministers and mandarins can go awry. So it is with the sensible-sounding scheme which posts police officers in schools to deter potential trouble-makers. In some cases, the Safer Schools Partnerships may have done just that. We do not know how many knife attacks, assaults on teachers, even murders have been prevented by the police presence.
We do know, however, that the scheme has had some unexpected consequences.
Inquiries by The TES this week show that more children are being arrested for offences that would be better dealt with by a head's sanction than a criminal record. One 12-year-old was reprimanded by the police after spending seven hours being questioned for shoving another pupil against a wall.
Already about 40,000 young people are being prosecuted for offences which 10 years ago would have been dealt with out of court, and young offenders institutions are overflowing. The partnerships appear to be adding to the numbers. Rod Morgan, head of the Youth Justice Board, admits that they are not always working as they should. More children are being criminalised.
They will leave school with records, struggle to find jobs and to escape from their criminal labels. Meanwhile, fear of the young - greater here than anywhere in Europe according to a think-tank report this week - will increase.
Some of the difficulties might be removed by adjusting the scheme.
Professor Morgan argues that full-time officers in a school reduce the number of arrests. Part-time officers, particularly unenthusiastic ones, may have the opposite effect. But a better solution to violent behaviour surely lies elsewhere. Another government programme, Every Child Matters, aims to bring together all those who deal with children, including the police, identify those who are at risk as early as possible and support them. Teachers are at its heart. Criminalising children for minor offences makes no sense. The seven hours spent questioning a 12-year-old could have been used more profitably - for her sake and ours.