SINGAPORE - On three different occasions, a teenager approached a stranger from behind and threw a plastic bag or sweater over her head.

Marcus Yow Kai Wen, 16, also covered a fourth victim's eyes with his hands.

On Monday, the secondary school student was placed on 18 months' probation for using criminal force on his victims, aged between 18 and 34, between March and May this year. All four incidents happened in the middle of the night.

Yow has to remain indoors between 9pm and 6am and perform 60 hours of community service. The court also ordered him to seek psychiatric treatment as required. His parents have signed a $5,000 bond to ensure his good behaviour.

The court heard that Yow committed one of his offences on May 13. Around 3am that day, he left his home in Choa Chu Kang with a plastic bag and wandered around the neighbourhood looking for a female "to disturb".

About an hour later, he spotted an 18-year-old and followed her. Yow then covered his victim's head with the plastic bag and pulled her towards him.

When she struggled, he held her by the waist and persisted in pulling her backwards. Yow fled after the victim pulled off the bag and screamed for help.

On May 17, having covered the lower part of his face with a red sweater, he hid behind an HDB void deck pillar and observed a 34-year-old talking on her mobile phone.

He then tailed the victim as she walked towards the lift lobby and covered her eyes with his hands. He stopped and fled home after she kicked him and screamed for help.

Yow pleaded guilty to two of four charges, with the rest taken into consideration. He could have been jailed for up to three months and fined up to $1,500 for each offence.