It was possible that Mondeor High School pupil Kulani Mathebula was killed for his cellphone, Gauteng police said on Thursday.

The 19-year-old, who was in Grade 11, was stabbed a short distance from his home as he was walking to school on Wednesday. 

Captain Kay Makhubele said the police's preliminary investigations had shown his killing could have been part of a botched robbery.

Before Mathebula's body was removed from the scene, forensic officials were seen emptying his bag and searching through his blazer as they recorded the items they found on him after the attack. 

"His phone was not in his possession," said Makhubele.

Three suspects have since been arrested. One is a 13-year-old boy, while the other two are 15.

Commenting on the younger pupil's arrest, Makhubele said: "Police formed a team of detectives who followed up the information from the scene, which led them to another school in Mondeor, where one 13-year-old suspect is a pupil."

The 15-year-old boys were arrested in Soweto and Naturena.

Police had earlier called on the pair to hand themselves over but Makhubele said they had instead been arrested at their homes in the early hours of Thursday morning.

It was not clear whether any of the suspects had known Mathebula prior to the attack.

The trio is expected to appear in the Johannesburg Magistrate's Court on Friday. Because they are minors, the proceedings will most likely be held in camera.

 A white cross has been erected at the park where Mathebula died.