A former relief teacher at a secondary school, was jailed for 18 months on Wednesday for having sex with a 13-year-old student.

Ross Ryan Kristiaensen, now 30, had admitted to two of four charges of having sex with the girl in December 2009 and October 2010.

Then a home economics teacher, Kristiaensen selected the victim to take part in a salad competition organised by a community centre in end-July 2009.

He took her out to buy items a day before the competition.

After the competition, they became close and would call or text each other daily.

He invited her to his flat on Nov 25, 2009. He hugged and kissed her and asked her to be his girlfriend. He also told her to keep their affair secret.

After having sex with her at his home on Dec 3, 2009, he promised that he would marry her. On the other occasion on Oct 10, 2010, she invited him over to her place as her parents were on an overseas trip.

Their relationship came to light two days later when her family members found her sitting very close to Kristiaensen in his room at Khoo Teck Puat Hospital where he had been warded. She was crying then.

In his oral grounds of decision, District Judge Mathew Joseph chided Kristiaensen for his "most unbecoming'' conduct as a teacher. He was supposed to be a role model as an educator. Instead, he was devious and manipulative and also told lies to further his selfish sexual desires, he said.

Kristiaensen could have been jailed for up to 10 years and fined on each charge.