It should be a matter of outrage that a man convicted of a sex-related offence can leave prison and a few weeks later walk into a school to take up a job working with children. In most jurisdictions in the developed world there would be safeguards. In Australia and the UK, for instance, all who work with children are subject to criminal checks. Yet in Hong Kong this can happen, as we saw last week when former policeman Wong Chi-ho pleaded guilty to nine counts of indecent assault on four girls aged 10 and 11 in the TV broadcast room of a Whampoa primary school. Two previous convictions for loitering in women's toilets, including a three-month jail sentence, had ended his police career but not employment in schools. On the same day, PE and Chinese teacher Chung Yui-hung admitted having sex with a 12-year-old girl he had lured through the internet. Six files of child pornography were also found on his computer. There is no reason to mistrust the vast majority of teachers and support staff. But two cases on the same day indicate paedophilia is as real here as in countries where there is far greater public awareness and concern about the risks children face. Neither the police nor Education and Manpower Bureau keep statistics on sex-related cases involving those who work with children. But child protection groups believe that those coming to court are the tip of the iceberg. Certainly, there are known cases of abuse by teachers who have been quietly moved on from their posts, no doubt to prevent a scandal. Schools are an obvious magnet for such criminals. The government owes it to children - all required by law to attend school - to ensure campuses are as safe as possible. The EMB says it monitors court cases involving teachers and can deregister those convicted of sex offences. This may provide some control. But the system breaks down when it comes to those hired from overseas, who include the thousands working on the native English-speaking teachers scheme, in international schools and tutorial centres. As part of their registration, they are required only to state if they have previous convictions, not prove through a police report that they don't. The fact that the EMB washes its hands of ensuring the suitability of the many others working in schools in non-teaching capacities is an even bigger loophole. The EMB leaves it up to schools to vet staff. But schools can only rely on a convicted sex-offender's honesty in owning up to his history - which no one with intent to offend again is likely to do. Last week's cases illustrate not only how real the dangers are but the urgent need for the government to keep a record of all sex offenders to which all institutions that serve children can refer.