Dareshan Moodley, 16, was stabbed to death and his brother Revarsheen, 18, is recovering in a hospital here after they were stabbed, beaten and left for dead in a street in the mainly-Indian suburb of Chatsworth.
It was the second attack on Dareshan.
He survived in the first stabbing by a fellow student in the school.
Friends who were with them managed to stop a passing motorists to go to the family home to summon the boys' father, Jayce Moodley.
Moodley told media that he believed the trouble started when a boy at their school confronted Dareshan for having spoken to his sister.
After a remonstration, the boy allegedly pulled a knife and stabbed Dareshan, prompting concerns from the community and education authorities about how the boy had taken a knife onto school premises.
Moodley senior said he had approached the school to lodge a complaint about the first incident.
Meanwhile, The Daily News, quoting its sources, said five alleged attackers have been arrested and one was still on the run.
This was the second incident in a week at a school in the province.
Last week, at a high school in Hopewell in Pietermaritzburg, north of Durban, two boys were stabbed in what is believed to have been a gang-related incident as well.