Chief Inspector Mo Siu-hei said upon receiving reports in August, officers conducted a thorough probe into the matter and found the allegations to be exaggerated and false.
Mo added that some of the video clips that came with the letters were recorded four to eight years ago.
“After investigating [the case], it was found that some of the accused staff in the clip was not present at the time,” he said.
He pointed out that officers had gone through all surveillance camera footage at the school between June and August this year, but found nothing suspicious.
“The letter is aimed at the school management and specific staff. We do not rule out that someone was dissatisfied with the work arrangements, and then made anonymous letters accusing colleagues and senior staff of child abuse,” he said.
The 48-year-old was arrested in suspicion of accessing a computer with criminal or dishonest intent, and wasteful employment of the police.
Officers said they seized a computer and printer from her office, which they believed were used for making the anonymous letters.