SINGAPORE - A teacher who headed his school's department for character and citizenship was sentenced to four years and nine months' jail on Monday for sexually abusing a 15-year-old student.

The teacher, who is now 33 and unemployed, had admitted to five of 10 charges of sex-related offences between September and December 2012.

The court heard that the local merit teaching scholar was the girl's form teacher in 2011, and by year-end, they had communicated with each other via WhatsApp text messages and over Facebook.

In 2012 - when he taught her literature - she started confiding in him about her family and her struggles with her studies after going on an overseas trip he led. He had a fiancee by then.

Subsequently, their communications became increasingly personal and the accused would engage her in conversations of a sexual nature.

On one occasion, he asked if she would have sex with him as a forfeit if she lost a bet with him. On another occasion, he asked her to send him nude pictures of herself and she complied.

On the forfeit, Community Court Judge Shaiffudin Saruwan said he did not think the question was asked innocently.

"Rather, it was part of a sexual grooming by the accused,'' he said.

He said the accused, who was 16 years older, had "overstepped his boundary of authority and exploited the trust of the victim'' by having an intimate relationship with her.

"There is a strong element of moral corruption of the victim by the accused,' he added.

The accused first took her to his parents' home in Kensington Park during the September school holidays and committed an obscene act by French kissing and caressing her breasts.

In October, they went to a secluded spot along Rifle Range Road where he had unprotected sex with her.

In mid-November, he took her to V Hotel Lavender where he sexually abused her and handed her, for the first time, a female contraceptive device he had bought overseas for his then-fiancee to use but the opportunity never arose.

They had sex at Kensington Park on Dec 10 when his parents were not at home.

In May last year, two weeks after their last sexual encounter, he got married.

His marriage affected her psychologically and she felt angry at herself and started to scratch herself until she bled. She sank deeper into depression, had suicidal thoughts and was warded in November for suicide risk and medical complications. She was later diagnosed with major depressive disorder, post-traumatic stress disorder and anorexia.

The offences came to light after she informed her therapists about the incidents. Her father lodged a police report in February this year.

The accused, whose head was bowed throughout the sentencing, could have been jailed for up to 10 years and fined on each charge of having sex with a person below 16 years. For committing an obscene act, the maximum punishment is a $10,000 fine and five years' jail.