Prosecutors are seeking 18 months' jail for a piano teacher with autism who was 19 when he molested a five-year-old pupil in a studio where he worked.

The man, now 22, had slipped his hand into her underwear and told her "remember this", and then scratched her, causing injuries that took two weeks to heal.

Deputy Public Prosecutor Gail Wong argued yesterday that while the offender was young, deterrence was the primary sentencing consideration as there was no causal link between his autism spectrum disorder and the offence.

"There was a real sexual motivation borne out of arousal; it was borne out of sexual intent," she told the High Court.

The DPP did not seek caning. She noted that people with autism may have hyper-or hypo-sensitivity to sensory input, which could make caning a more disproportionate punishment for him.

The defence, on the other hand, pressed for probation, arguing that rehabilitation should be at the forefront for a young offender.

"If he was to go to jail, I don't think he can survive," said Mr Shashi Nathan, noting that the man's parents have taken care of various aspects of his life.

Justice Pang Khang Chau adjourned the case to find out why the man had been exempted from national service.

The judge also asked for a report by a Ministry of Social and Family Development (MSF) psychologist to be shown to two psychiatrists who had earlier said there was a link between the man's actions and his autism.

In the MSF report, the accused disclosed that he was fantasising about a pornographic scene while molesting the victim. He also admitted to watching pornography - which he had previously denied.

The man, who cannot be named owing to a gag order to protect the victim's identity, pleaded guilty in April to a charge of molesting a person under the age of 14.

The court heard that he had started taking piano lessons from the owner of the studio in 2013.

After he completed his piano diploma and his GCE Normal (Academic)-level exams in 2015, his father arranged for him to teach piano part-time at the studio.

On Nov 7, 2016, while he was alone with the victim as usual during a lesson, he told her to sit properly.

He told her that he had to correct her posture while she was playing the piano, then slipped his hand inside her underwear and scratched her around her genital area.

She repeatedly uttered "ouch" and told him to stop, but he continued while apologising. The girl, who did not know her private parts should not be touched without her consent, did not tell her grandfather what happened when he came to pick her up.

At home, when she took an unusually long time in the toilet, her mother went to check on her and found her crying in pain.

The girl then revealed what her teacher had done to her, and her father called the police.

After the incident, the girl had several episodes of bed-wetting and was seeing a counsellor.

The DPP sought a compensation order of $7,000.

Mr Nathan said his client had no issue with paying for the victim's counselling sessions.

However, he took issue with a victim impact statement submitted by the prosecution, in which the girl said she would not be able to enjoy her upcoming birthday if the man was not jailed.

Justice Pang agreed that while he had to know how the victim suffered, it was inappropriate to "exert emotional blackmail" on the court.