They had sex two more times the following month.
On Tuesday, the tall youth, who wore a long-sleeved light grey shirt and black pants, was visibly nervous before his case was mentioned in court.
His father and aunt sat behind him. The 23-year-old trainee teacher pleaded guilty to one charge of sexually penetrating the minor and agreed to have the other two charges be taken into consideration.
His lawyer, Mr Amolat Singh, told the court in his mitigation that the accused, then an 18-year-old polytechnic student, met the girl on an online chat.
Within two weeks, they exchanged phone numbers and SMSed each other.
They met for the first time at West Mall four weeks later. He sensed that the girl had a crush on him and they later went to the highest floor of a HDB block where the girl hugged him at a staircase.
They engaged in some "dirty talk" in Mandarin and she took his hand and rubbed her private parts with it.
After that incident, they did not meet for more than a year but kept in touch through text messages.
It was during this period that he got involved with a 12-year-old girl.
In April 2008, the accused, then a 20-year-old full-time national serviceman, met the 15-year-old girl again at West Mall.
She asked him if he would touch her again and he replied that they were in a public place. They then went into a toilet for the handicapped on the fifth floor.
Inside, the girl hugged and kissed him before undressing. He also undressed and they had sex.
They had sex two more times the next month.
On June 26, she lodged a report against him at a neighbourhood police post. It was not mentioned in court why she did so.
Deputy Public Prosecutor (DPP) Sellakumaran said that the defence seemed to suggest that the victims had seduced the accused.
The DPP earlier told the court that the accused had first asked the girl for sex in March 2008 but was turned down. She later changed her mind in April.
Mr Singh said the fact that the girls were underage remained the crux of the case and the accused was aware of their ages.
He said he had explained to his client that the "disposition of the victims is irrelevant to the charge".
Urging the court to impose a light sentence, Mr Singh said that his client would be sacked from the National Institute of Education and have to repay $50,000 to $60,000 for breaching his training contract.
District Judge Hamidah Ibrahim said the "characters of the two victims are totally irrelevant".
She said the accused's five charges and the victims' young age were aggravating factors against him.