A 35-year-old former teacher who sexually assaulted a 12-year-old girl nearly two years ago was jailed for 21 months yesterday.

Du Tao, a Singaporean, pleaded guilty to two charges - sexual penetration of a minor and committing an indecent act with a child.

He admitted to two other similar charges which were taken into consideration during sentencing.

A district court heard that Du, who was then 33 and teaching Chinese to Primary 3 pupils at a school in the north, got to know the girl on online photo-sharing app Mei Pai. She had posted her username to her WeChat account.

The girl told Du that she was 13 and sent him photos of herself in her school uniform. She called him "Daddy", while he called her "Nu Er" - daughter in Chinese.

Du's chats with the girl became explicitly lewd, and he told her he would teach her how to have sex.

On March 8, 2014, a week after Du got to know the girl, she invited him to her flat.

While she was having her lunch there at about noon, he suddenly hugged her from behind. She stood up and turned to face him, and they started kissing.

Du led her to the bedroom and they continued to kiss on the bed, before he sexually assaulted her and made her perform a sex act on him.

Shortly after, the police, responding to a tip-off, knocked on the door.

Deputy Public Prosecutor Selene Yap asked for a total of 21 months' jail, noting that Du had kept in constant contact with the girl for a week before meeting her, and sent lewd messages.

But Du's lawyer Jenny Lai asked for 18 months' jail. She said her client, who has a newborn son, has been unemployed for some time, and had "a remarkable record in the teaching profession".

District Judge Eddy Tham said Du's messages to encourage the girl to continue with explicit sexual words and innuendoes clearly showed moral corruption. He had also exploited her naivety and actively preyed on her. If not for the timely arrival of the police, his sexual assault would have worsened.

But the judge said there was no evidence Du had trawled online for a young victim. And while Du's occupation as a teacher might cause some public disapprobation, he did not commit the offences in his capacity as a teacher nor in the course of his work.

A spokesman for the Education Ministry said yesterday that Du had left the service shortly after the case was reported.