The incident happened in Tindal Street near Langton Road, Camberwell at around 1pm on August 10.
Officers and London Ambulance Service crews attended and found an 18-year-old boy who had been stabbed.
He was taken to hospital where his condition was assessed as not life threatening.
Two boys, 14 and 15, were arrested on suspicion of grievous bodily harm the following day.
The 15-year-old boy was charged on August 11 with grievous bodily harm with intent and possession of a knife blade in a public place.
He appeared in custody at Bromley Magistrates’ Court on August 12.
He has since been bailed to next appear at Bromley Youth Court on September 3.
The 14-year-old has been bailed to return on a date in mid-October.
Police enquiries are ongoing.
Anyone with information is asked to call police on 101 with the reference 3350/10Aug.
