Eliza Reberio, a sales assistant from Croydon, was just 14 when she set up Lives Not Knives after seeing a number of friends get stabbed in gang conflicts.
Eliza, now 17, said she was "suddenly faced with a very different side to life" following her expulsion from Coloma Convent girls' school for bad and disruptive behaviour in 2006.
"I saw and heard about friends being stabbed and I thought it was normal," she said.
"I would get texts saying someone had been stabbed the night before and I wasn't shocked. Then I looked at my friends' little brothers and sisters waking up to those texts and I wanted to change things."