Mayor of London Boris Johnson said:
"It is heart breaking to see the life of another young Londoner taken so tragically. My thoughts are with this young man's family at such a terrible moment."
"This horrific incident serves as an unwelcome reminder that although violent crime in the capital continues to fall, we must continue our fight to tackle youth violence and get knives off our streets."
More top news
A "weather bomb" is set to bring heavy wind and rain to the UK over the weekend.
Aydin Onac has been suspended from his role at St Olave's Grammar School in Orpington.
Leaders of the Aslef union are recommending that their members accept the deal.