Lo and Chan Chin-hong, Form Four students, pleaded guilty yesterday to a joint charge of wounding with intent on August 25 outside Lei Muk Shue Estate.

Lo also pleaded guilty to a separate charge of common assault on the boy a month before the chopping attack, in which he kicked the boy in the back at the same site.

The court heard that the same man, identified only as Ah Foo in the prosecution's summary of facts submitted to the court, hired Lo to carry out the two attacks.

Lo told the court Ah Foo refused to pay for the first job. Ah Foo had told Lo that the mastermind of the attack, Shum's 21-year-old stepmother, Hung Man-yee, was 'discontented' with the first attack because Ho-yin 'did not suffer any injury' and therefore she had refused to pay.

Hung's bank records show an electronic transfer of $2,500 from her account to Lo's account three day before the chopper attack. Further cash deposits were made into Lo's account a day after the attack.

Sentencing of the two students' was adjourned until the conclusion of the trial of four co-accused.

Hung has been charged with wounding with intent and common assault on her stepson. Her case is pending transfer to the High Court, where she will enter her pleas. The trial of the other three co-accused - Tsang Ho-wai, 23, the alleged ex-boyfriend of Hung, and two taxi drivers, Ng Tak-wing, 45, and Yeung Sing-kwong, 25 - will begin on July 19.

The prosecution said Ho-yin was often scolded by his stepmother, who had repeatedly urged her husband, who she married in 2004, to return Ho-yin to his mother or send him to the Po Leung Kuk.

Hung gave birth to a boy in February last year. Ho-yin and the baby boy had both been in the full-time care of Ho-yin's 67-year-old grandmother, the three living together in the same public housing flat.

Senior government counsel Anthea Pang said that on both the days Ho-yin was attacked, Hung had unexpectedly turned up at the flat and told the grandmother she wanted to be with her baby. The grandmother was then relieved from carrying the baby with her when she took the boy to school and picked him up after school, as she usually would.

The prosecution alleged Lo recruited his school friend Chan and paid him $1,000 to assist him in the chopper attack. Chan, who was also armed with a chopper, had grabbed Ho-yin's grandmother when she was on her way home with the boy from a tutorial at Lei Muk Shue Estate at about 5.30pm on August 25 last year. Lo held the boy on the ground and chopped his right forearm and right thigh several times. The pair confessed under caution when arrested, one in Hong Kong and one on the mainland, in September.