A High Court judge yesterday said it was ironic that a man who had barged into a primary school classroom and slapped a nine-year-old boy for allegedly bullying his daughter then proceeded "self- righteously" to lecture the entire class on how it was wrong to bully others or behave like gangsters.

Justice See Kee Oon noted this as he sentenced the 47-year-old father to two weeks in jail, following an appeal by prosecutors that the initial sentence of a $3,500 fine by a district judge was too lenient.

The man, who cannot be named to protect the identities of the children involved, had pleaded guilty to causing hurt to the boy, who was so traumatised that he wet himself after being slapped and berated.

Justice See agreed with prosecutors that a jail term was warranted to address the need for deterrence, the harm caused and the culpability of the man, who is a hawker.

He disagreed with the district judge's conclusion that a fine was sufficient because the man had acted on the spur of the moment and that the injuries were slight.

"What is especially aggravating is the respondent's deplorable violation of the sanctity of the school environment and the distress and disquiet caused to the children and their teacher," said Justice See.

He noted that the district judge viewed this as a case of the man trying to protect his daughter, albeit in the wrong way. "With respect, this cannot possibly be a relevant factor in mitigation when the 'protection' took the form of striking a vulnerable and defenceless nine-year-old child," he said.

The man, who told the judge he needed to earn more money to pay his children's school fees, will be allowed to start his jail term on Aug 10. The fine he has paid will be refunded.

On the morning of Oct 7, 2015, the man's daughter said she did not want to go to school because the classmate liked to "disturb" her. Her father drove her to school and was allowed into the compound after he told the security guard he wanted to see the principal.

Instead, he went to a classroom on the third floor and interrupted a music lesson. The girl pointed out the classmate, and the man slapped the boy's left cheek when the child denied the accusation.

The teacher then pulled the boy behind her to shield him, but the man continued hurling accusations at the victim.

Yesterday, Deputy Public Prosecutor Christina Koh argued that the man's "brazen excuses" showed a lack of remorse.

The man had contended that security at the school could have been tighter and that the incident had served as a life lesson to the children. He attributed red finger marks visible seven hours later on the boy's face to "sensitive skin".

DPP Koh argued that there was a clear public interest in deterring people from intruding into schools to attack vulnerable victims.