Jimmy Mizen, an altar boy at a nearby Catholic church, died the day after his 16th birthday in an incident at a baker's in Lee Green, South-East London.
He had called in at the shop on Saturday morning with older brother Harry, 18, after buying his first Lottery ticket.
His devastated father Barry, 56 - a governor at St Thomas More School in Eltham where Jimmy was a pupil - said: "He wanted to play the lottery for the first time because he'd just turned 16. It was part of his birthday celebrations.
"It was mindless violence, just pure aggression.
"A fellow picked up a glass dish and smashed it against the side of his neck. We're told he bled to death. My other son Harry was with him.
"He is so upset. He said the blood was just pouring out of him, they just could not stop it. I just can't believe it."
A witness, who asked not to be named, said she saw an "unprovoked attack" on Jimmy. She said: "It looked like something out of a a Tarantino movie."
Jimmy's mother Margaret, 55, paid tribute to her son – one of nine children – who played for Sidcup rugby club.
She said: "Jimmy was the loveliest, gentlest giant. He was loved by everyone, by his school, the church and his family."
His brother Tommy, 24, rushed to the scene after being called by Harry, and was cradling Jimmy in his arms when he died.
Six-foot-tall Jimmy was due to go on a trip to Disneyland California as a birthday present from his parents and then was set to begin an apprenticeship with a local builder.
Father Edward Perera, of Our Lady of Lourdes Church, where Jimmy was an altar boy, described the Mizens as pillars of their community.
"They are a lovely family, God-fearing and God-loving, and Jimmy was a lovely, helpful boy," he said.
"They are all totally distraught and devastated and can't believe that this could happen to anyone."
Investigating officer Cliff Lyons said: "Our sympathies are with his family and friends. At this early stage it is understood that Jimmy was in the Three Cooks Bakery when the young male suspect entered the premises.
"There was an argument, following which Jimmy was assaulted. He sustained serious neck injuries."
