The Primary Two student, identified only as Yuen, was reading when 10-year-old Law Cheuk-kei fell from the fifth-floor stairway at CCC Kei Chun Primary School in Kwai Chung on Monday morning.
The Primary Five student was found lying unconscious on the ground by teachers.
She was given first aid by teachers before an ambulance was called.
The school had not received any report that Cheuk-kei was being bullied. But Yuen said her friend was often picked on and felt unhappy about going to school.
Police are investigating the case and interviewing students. Officers believe other students may have seen what happened, but are afraid to speak up.
School principal Wong Ching-man faced criticism yesterday that the school did not dial 999 straight away. She said the school had followed guidelines set out by the Education Bureau for handling such a crisis.
But education lawmaker Ip Kin Yuen found it strange that the school did not call the police immediately. He said the Education Bureau guidelines for situations like this were very simple. He said the teachers should have used common sense.
Cheuk-kei was found at 7.40 am. The school called St John Ambulance at 7.46 and an ambulance arrived 11 minutes later.
Paramedics notified the police, and a Fire Services ambulance arrived at 8.19 to take Cheuk-kei to Prince Margret Hospital, where she was declared dead. Cheuk-kei's parents did not comment on the school's actions.