A group of seven teenage girls launched an attack on a 15-year-old last December, slapping her in the face repeatedly to "teach her a lesson", a court heard.
One of the seven, Anna Belle Tan Yee Min, now 17, also burnt the victim's thighs with a lit cigarette.
Yesterday she was sentenced to 21 months' probation for her role in the assault.
Another 17-year-old, Nisha Wong Hui Xian, was previously sentenced to probation while the other five, aged between 13 and 16, got police warnings.
It was not said what provoked the attack, but a community court heard that at about 3pm on Dec 20 last year, Tan and her six friends went to look for the victim - who has not been named - at the void deck of Block 231, Hougang Street 21.
All seven took turns to hit the victim in the face. One of the assailants also pushed her to the ground.
The group fled after warning the victim not to inform the police, but she made a report on Jan 11.
Later that month, at about midnight on Jan 31, Tan and another assailant, aged 13, saw the victim at a Cheers convenience store in Hougang Central.
Tan asked the victim whether she wanted to "drop the case" against her, the court heard.
Shortly after, the victim called her mother who then warned the two assailants not to assault her daughter.
However, Tan splashed a bottle of iced lemon tea at the victim and slapped her. She also warned her not to report the matter to the police, before leaving.
The court also heard that at about 1pm on Dec 20 last year, Tan slapped a 14-year-old girl at the carpark of Nex shopping centre.
Fifteen minutes later, she slapped another 14-year-old girl at the fourth-floor staircase landing of the shopping centre. These two charges of causing hurt were considered in sentencing.
The maximum penalty for rioting is seven years' jail with caning for men below 50; and for causing hurt, two years' jail and a $5,000 fine.