Before he and his group targeted foreign workers to beat up, he took up martial arts lessons.
After identifying weaker and smaller targets to test their fighting skills, the four teens attacked two foreign workers on separate occasions.
Daryl Lim Jun Liang even mocked the victims by dancing in front of them and pushing one of them on the head.
He and his friends then fled, leaving the workers injured and bleeding.
In January, he pleaded guilty to one charge of voluntarily causing hurt, with another charge to be taken into consideration during sentencing.
Calling the offences heinous, Deputy Public Prosecutor (DPP) Nicholas Lai yesterday called for Lim, 18, to be sent for reformative training, which ranges between 18 months and three years behind bars.
But District Judge Lim Keng Yeow felt reformative training was too harsh and instead sentenced the Institute of Technical Education student to a 10-day short detention order - a community sentencing option that is intended to be less disruptive and stigmatising than jail.
Lim was also placed on a 12-month day reporting order with tagging and ordered to perform 150 hours of community service. The day reporting order means he would have to remain indoors from 10pm to 6am.
The teenager is out on $15,000 bail after the prosecution applied for a stay in execution of the sentence while it reviews whether to file an appeal.
Lim's co-accused were Tan Jun Liang, 18, and two 15-year-old boys who cannot be named because of their age.
On Oct 3 last year, the four teens assaulted Chinese national Zuo Yu Nian, less than a month after they bashed Indian national Selvaraj Madankumar.
Last month, DPP Lai had asked for Lim to be given 18 months' probation but yesterday he urged the court to impose the tougher sentence of reformative training.
"A deterrent sentence is needed more than ever in light of the increase in the number of youth crimes involving violence," he said, noting a 2.1 per cent rise in the number of youth arrested from 3,031 in 2013 to 3,094 last year.
A total of 322 youth were arrested for rioting last year, a 13.8 per cent increase from the 283 in 2013.
In his submissions, DPP Lai said Lim had learnt martial arts at a gym for about three to four months before he and his friends went looking for targets to practise their martial arts on.
He said the offences were particularly heinous with a high degree of premeditation as the victims were targeted because they were weaker and unlikely to fight back.
He added that Lim had claimed to have played a passive role in the assault and that one of his younger co-accused was the mastermind.
"Can we really believe that a 15-year-old negatively influenced him to assault the victim?" DPP Lai said.
The 15-year-old boys were each sentenced to 18 months' probation in February. Tan's case is pending.
DPP Lai said Lim, as the second oldest member of the group, warranted a greater legal and moral culpability.
"(Lim) is not a juvenile and ought to have acted with maturity. This means that he ought to have dissuaded the group from even embarking on the path of destruction," said DPP Lai.
"There is now a more-than-ever pressing need to send a clear message to potential offenders that committing acts of gratuitous violence against others will be met and countered with the firm hand of the law."
DPP Lai added that Lim, who attended Yishun Secondary School, was said to have "lacked purpose in life" and had committed various disciplinary offences.
In his probation report, Lim told the probation officer that he was part of the Ang Soon Tong gang from late 2013 to early 2014. He attended one gang funeral and a "settlement talk", leaving the group only after two of his leaders were arrested.
His lawyer, Mr Luke Lee, said his client was offered a job after graduation by Swensen's following a work attachment programme.
His parents told the probation officer that he was well-behaved at home.
But DPP Lai said: "If he is generally well-behaved at home... Why is he hanging out with unsavoury characters... and gone on to commit the offence?"
He said reformative training would expose Lim to various programmes that would help integrate him back to society. It would also distance Lim from negative peer influence and introduce him to an environment that would allow him to better focus on his future endeavours, he added.
Those sentenced to reformative training must undergo a strict regimen that includes foot drills and counselling.
But Judge Lim noted that Lim had a clean record and had been well engaged in school and his part-time work.
He added that Lim was receptive to parental advice and supervision, and has a warm and supportive family.
The probation officer found him not to have "deep-seated delinquent habits" and Lim was also assessed to be at low risk of reoffending, the judge said.
Even if Lim was dealt as an adult offender, he noted that an appropriate sentence would have been about two months' jail. Therefore, reformative training with a minimum 18 months' detention, would be too much of a sledgehammer approach.
It was important to note in deciding between reformative training and community sentencing that the former results in an "indelible criminal record", Judge Lim added.