A Maris Stella High School pupil, who was on his way to school on Tuesday morning, was found to be unresponsive on the school bus when it arrived at the school.
The Primary 5 pupil was taken by ambulance to the KK Women's and Children's Hospital (KKH), where he was pronounced dead.
The parents of some of his classmates said their children had told them the 11-year-old boy had been unwell the day before he died.
One parent, who declined to be named, said she received an e-mail from the form teacher addressing concerns of parents.
The form teacher wrote that pupils "may not be able to cope with the sudden loss of a classmate and a friend", but reassured parents that the school would offer "that listening ear to the boys and provide the emotional support that they need during this difficult time".
The e-mail also noted that subject teachers would be observing the class to check if there are any pupils who need help in managing their emotions. Parents, too, were encouraged to monitor their children to see how they were coping.
The school's principal, Mrs Woo Soo Min, said the school community was "saddened by the passing of one of our students".
"We are in touch with the family and are providing assistance and support to them during this difficult time," she added. "We are also providing emotional support to affected staff and students."
On Tuesday, the police received a call at about 6.50am requesting assistance at 80 Bartley Road, where the primary section of the high school is located.
They found the boy unconscious when they arrived. He was taken to KKH, where he was later pronounced dead at 8.46am.
The police are investigating the unnatural death.