The 14-year-old died after he was stabbed around 27 times on the route 472 at Woolwich Church Street just before 2.30pm on January 7.
Kelyan, an aspiring rapper, had sustained a severed femoral artery and died shortly after medics arrived at the scene.
Two boys, aged 15 and 16, were charged with Kelyan’s murder.
On Monday, the defendants, who cannot be named for legal reasons, were appearing at the Old Bailey for a preliminary hearing.
The 15-year-old appeared first by videolink from Feltham Young Offender Institution and spoke only to confirm his identity.
Prosecutor Tom Little KC told the court the victim was sitting on the back seat of the bus when he was attacked by two youths both armed with “lengthy machetes”.
He said: “It is clear this is not a form of spontaneous incident. The two defendants must have known the deceased was on the bus.
“They approach him and almost instantaneously the two of them pull out machetes and attacked the deceased.
“He is stabbed or attempted to be stabbed on a total of 27 occasions before the defendants made their way off the bus.”
One of the machetes was allegedly discarded in the River Thames afterwards.
During the hearing, Judge Mark Lucraft KC set a plea hearing for April 14 and a provisional trial at the Old Bailey from October 13.
He remanded the younger defendant into custody.
Later, the 16-year-old appeared in the dock of Court One of the Old Bailey and confirmed his identity.
Judge Lucraft confirmed the timetable for the case and remanded him in custody too.
Members of the victim’s family sat in court for the short hearings.
