A 15-year-old boy has been arrested on suspicion of the murder of Tamim Ian Habimana, also 15, in Woolwich, south-east London, on Monday evening.
A spokesman for the victim’s school said students and staff were “devastated” to hear of the “tragic and senseless loss of a child’s life.”
Meanwhile, a 29-year-old man was arrested on suspicion of the murder of Keane Flynn-Harling, 16, in Lambeth, south London, on Monday night.
The Metropolitan Police said the families of both victims are being supported by specially trained officers.
The number of violent teenage deaths in London this year could be one of the worst in nearly a decade.
As of Tuesday, a total of 21 teenagers had been killed in just over six months this year in the capital, leading to concerns that the grim tally for the whole year will exceed the previous high of 27 in 2017.
This would mean the highest teenage death toll since 2012.
Officers were called to reports of a stabbing in Woolwich New Road shortly after 5:20 p.m. on Monday and found Tamim suffering from a single stab wound.
He was pronounced dead at the scene at 6:08 p.m., despite the efforts of emergency services to save him.
Detective Chief Inspector Richard Leonard, who is leading the investigation, said the teenager’s family is “reeling from the shock of his death.”
He said Thursday’s arrest “marks a significant development in our investigation.”
He appealed for anyone with information that could help the inquiry to come forward.
He added: “We are aware of footage relating to this murder being posted on social media and remind the public that any video captures or images may be very important to our investigation.”
He encouraged people to upload anything relevant to the police here.
The teenage suspect was taken into custody at a police station in south London on Thursday.
A spokesman for Harris Academy Greenwich said: “Students and staff were devastated to hear that Tamim Ian, a student at our academy for the past two years, was killed on Monday evening in Woolwich. This is a tragic and senseless loss of a child’s life.
“Tamim had been one of our students for the past two years and would have sat his GCSEs next year. Our thoughts and condolences are with Tamim’s family and we will do everything we can to support them at this time.”
Separately, the Lambeth stabbing saw officers called to Oval Place at 11.45pm on Monday.
The victim, 16-year-old Keane, was pronounced dead at the scene.
The suspect in that murder remains in custody at a south London police station, the Met said.