Police are appealing for witnesses after a 16-year-old boy was found in Porlock Street, near Long Lane, suffering from stabbing injuries.
Emergency services were called to Porlock Street just after 3pm on Wednesday.
The teenage boy, believed to be aged 16, was taken to a London hospital where he is in a critical condition.
It is understood that the victim had just left the Porlock Hall Pupil Referral Unit, an education centre for young people excluded from local schools, when the attack took place.
"We believe there may have been a number of witnesses to this attack, either people coming home from schools in the area or general passers-by," says DI Bernie Galopin of Southwark CID.
"We would urge these people and anyone else who can provide information to contact us in the strictest confidence.
"Please call the incident room on 020 7232 6145, Crime Stoppers on 0800 555 111 or Southwark Police on 020 7378 1212."
The street remained sealed off late on Wednesday night.