A 15-year-old boy with a heart condition died after collapsing during a 2.4km run in school on Wednesday afternoon.

Teachers had tried to resuscitate Bryan Chee, a Secondary 4 student at Loyang Secondary School, but were unsuccessful, said his mother, Madam Cynthia Lee, 40.

Madam Lee, a billing associate at an IT firm, told The Straits Times that the school called her at around 5pm to say her son had collapsed.

She added that Bryan's teachers and classmates told her he had been on the first or second lap of the run when he suddenly sat down and collapsed on the track.

The Singapore Civil Defence Force said it was alerted to a call for an ambulance at the school at about 4.40pm. The police said the boy was taken unconscious to Changi General Hospital and later pronounced dead. The police are investigating the unnatural death.

While she is not sure of the exact cause of Bryan's death as the coroner's report will take three to five months, Madam Lee said his death certificate stated cardiac failure.

She said Bryan suffered from a congenital heart defect where his heart was missing a valve, though the affable and mature boy never complained of discomfort and even joined the school's basketball team.

She and her husband, Mr Larry Chee, 43, found out about their son's condition when he was seven.

After annual hospital check-ups for three years, his doctor said his condition was stable and they went for check-ups only every two years.

Madam Lee said that about 20 schoolmates visited Bryan's wake in Pasir Ris yesterday.

"They shared with me how likeable Bryan was and his friend's mother even said she treated him like her own son whenever he went over to their house," she said.

Madam Lee told ST that she hoped to perform religious rites at the school's sick bay, which is where she believes Bryan died. ST's attempts to contact the school's principal were unsuccessful.

Doctors told ST that heart conditions vary in risk, depending on the severity of the problem.

Cardiologist Dinesh Nair at Mount Elizabeth Hospital said: "Even with frequent check-ups and tests, there's no 100 per cent pick-up rate on how severe the condition might be.

"The only thing doctors can do is to reduce the incidents and risks with proper check-ups, assessments and advice."