A police officer who posed as a secondary pupil to catch students recruiting for Triad gangs almost blew his cover when he came top of the class in school exams.
The six-month covert operation code-named "Gigshill", carried out in co-operation with the principal, has raised new concerns about Triad activity and violence in Hong Kong schools.
Triads are involved in extortion, racketeering, prostitution and drug dealing and operate like secret mafia gangs. It is feared that they are expanding their activities in schools.
Senior inspector Simon Chan Wai-ming of the Tuen Mun anti-Triad squad said: "One of our agents got excellent results in an exam. Another student, previously the top pupil, complained to the assistant principal that the results were too good to be true."
Despite this, within a month the 19-year-old officer was approached by a student at the school in the new town of Tuen Mun to join the Sun Yee On Triad society. A fellow officer was asked to become a member within a day of starting school.
The Gigshill operation culminated in the arrest of 25 people, 24 of them students at the school.
Both agents developed detailed cover stories about their backgrounds before being planted in the school. Each night they returned to a safe-house to do their homework - making notes on the evidence they gathered during the day.