Keon Lincoln, 15, was shot in the stomach then repeatedly knifed while on the ground in Handsworth, West Midlands, in broad daylight in January.
The 14-year-old, from Birmingham, who cannot be named because of his age, stood with his hands clasped and showed little emotion as he was unanimously convicted.
A 16-year-old, from Walsall, whose identity is also protected by a court order, was found guilty of murder after being caught on CCTV armed with a large knife.
Tahjgeem Breakenridge, 18, from Balsall Heath, and Michael Ugochukwu, 18, of Edgbaston, were also found guilty of murder after 15 hours and 50 minutes of deliberations.
The 14-year-old, from Birmingham, who cannot be named because of his age, stood with his hands clasped and showed little emotion as he was unanimously convicted.
A 16-year-old, from Walsall, whose identity is also protected by a court order, was found guilty of murder after being caught on CCTV armed with a large knife.
Tahjgeem Breakenridge, 18, from Balsall Heath, and Michael Ugochukwu, 18, of Edgbaston, were also found guilty of murder after 15 hours and 50 minutes of deliberations.
The 14-year-old was also convicted of possession of a firearm with intent to endanger life while the 16-year-old, Breakenridge and Ugochukwu were all convicted of having an offensive weapon.
Jurors will resume their deliberations on 18-year-old Kieron Donaldson, who denies murder, tomorrow.
