Judge, David Radford warned Mary Konye that she will face jail when he sentences her on March 7:
I should make clear that, in my judgment, this is a case that will, in all likelihood, need a substantial custodial sentence. It is inevitable, but it won't be dealt with until I have received those reports.
Victim, Naomi Oni burst into tears as she left the courtroom and was hugged by family and friends.
More top news
Paramedics attended a scene in Willesden, north-west London, on Monday afternoon where they found the teenager with stab wounds.
At least five people have suffered minor injuries but the cause of the blast is still unknown.
A songwriter from Harlesden who wrote a track for Beyonce's Lemonade album has been nominated for a Grammy Award.