Quamari Serunkuma-Barnes was stabbed in the chest outside his school, Capital City Academy in Willesden, on Monday afternoon as he was on his way home.
A 15-year-old boy has been charged with his murder.
At the vigil, parents were urged to stop being afraid of their children and to check their rooms for weapons.
His cousin Rachel Odunton said: "He wasn't in a gang. He wasn't running the streets. He went home and did his homework. I miss him so much.
"We need to get people off the streets now, it's not safe. Quamari wasn't involved in any of this. He was a good boy and he was humble and he loved everybody in his family. He was a special boy, he wasn't just anybody so this sort of thing needs to stop now.
"Wherever you are from it doesn't matter. We are one community and we need to join together now because this matters."
At the event, black and gold balloons were released in remembrance to Quamari.
In front of about 300 people, friends and community leaders spoke about the need to bring the community together.
His relative Sylvia Tella said: "We are not taking control. This is something that has to be changed. The police, no matter what they do, unless we help, they can only do so much."
She added: "You do not expect to hear that your child has been stabbed outside his school.
"This is not a black life, this is all lives."
A family friend said: "We need to take some responsibility for our children. We need to stop being afraid of our children."
Last year, 12 teenagers were stabbed to death in the city.
Family and friends of a murdered 15-year-old boy were among hundreds of people who gathered for a memorial to him.