Friends of a murdered teenager faced a sombre return to school today on what would have been his last day as a pupil.

Jimmy Mizen, 16, was murdered in an unprovoked attack after refusing to take part in a fight in Lee, south-east London, around 11.50am on Saturday.

Police said last night a key witness, a white middle-aged man who was in the shop at the time, had come forward after a public appeal.

Jimmy was near his home in Three Cooks bakery when a youth unsuccessfully challenged him to a fight.

The attacker then smashed down the shop door and picked up an advertising board which he wielded inside.

"Gentle giant" Jimmy, who had gone out to buy his first Lottery ticket after celebrating his 16th birthday on Friday, died from fatal neck injuries at the scene in front of his brother.

He is the 13th teenager to be violently killed in London this year.

Today school friends, many of whom have paid tribute to Jimmy, returned to St Thomas More Catholic Comprehensive in Eltham for their last day in Year 11.

His father Barry, 56, vice-chairman of governors at the school, described his son as a "gorgeous and loving boy" with a sense of decency who adored his brothers and sisters and "wasn't afraid of hugs".

Mr Mizen, who runs a shoe repair shop in Sidcup, said: "We loved him and he loved us back. He was very proud of his family."

His wife Margaret, 55, said: "He was the loveliest, gentlest giant. He was loved by everyone, by his school, the church and his family."

Detective Chief Inspector Cliff Lyons said the attack on the 6ft 4in teen was not gang or knife-related.

Yesterday around 250 people left standing room only for Mass at Our Lady of Lourdes Catholic Church in Lee, where Jimmy had been an altar boy as a child and had been baptised.

A two-minute silence was held at the service, which was attended by Jimmy's parents.

One of the altar boys at the Mass was Jimmy's eight-year-old brother George.

Jimmy's other brothers included Danny, 30, Billy, 28, Tommy, 26, Bobby, 24, and Harry, 18, and two sisters Joanne, 35, and Samantha, 21.

Parish priest Father Edward Perera said the service had been deeply overwhelming for him. He said: "I have never seen a parish so numbed and shocked."

Conor O'Hehir, 16, who had known Jimmy since the age of three, said: "He was a brilliant lad and friend and he's going to be greatly missed."

Around 50 family and friends, including Jimmy's parents, were allowed beyond the police cordon to lay flowers and read messages of condolence about five metres from the bakery shop where the killing happened.

Tributes flowed for Jimmy from family and friends.

His brother Danny said: "Jimmy was the life and soul of the family.

"If you asked any of the brothers and sisters I'm sure they would say Jimmy was their favourite.

"He was just a good guy, really enthusiastic in everything he did and always had time for all of us."

Jimmy's friends posted tributes to him on networking website Facebook.

St Thomas More pupil Jack Tatlock said: "I can't believe what has happened mate! You should still be here with us! I miss you already!"

Another schoolmate, Charlie Jennings, wrote: "I just can't believe you're gone, it will never be the same in school without seeing you around. You didn't deserve to leave so early in your life."

Police urged anyone with information about the killing to call 020 8721 4205 with information, or Crimestoppers anonymously on 0800 555 111.