The victim was found injured in Doyle Gardens, Kensal Green, just before 15:30 GMT on Monday.
Ambulance crews treated the boy at the scene and took him to hospital where he was later pronounced dead.
Metropolitan Police murder detectives have yet to release the boy's identity but said his next of kin had been informed. No arrests have been made but witnesses are being sought.
Capital City Academy in Willesden has confirmed the stabbed teenager was a pupil at the school.
A friend of the victim, who did not want to be named, told BBC London Radio: "He was one of the most loved people in the whole of our school.
"I gave him hug and said, 'Get home safely', and then I come out of school and I see that he's stabbed.
"There's all these teenagers out here running with knives and guns. What's the point? Where does it lead us?"
A 15-year-old boy has been stabbed to death in a London street.