Mr Lawrence, 48, was stab-bed in the chest in a "deliberate, concerted and cowardly attack" by the 15-year-old leader of a street gang "acting with a mixture of bravado and adrenalin", the court heard.
The teenager, who cannot be identified for legal reasons, appeared charged with Mr Lawrence's murder yesterday on the first day of the trial in court number one of the Old Bailey.
The youth denies the charge and also denies a second charge of conspiracy to cause grievous bodily harm and wounding with intent a 13-year-old boy. A co-defendant, aged 15, also denies the latter two charges.
John Bevan, for the prosecution, told the jury of six men and six women that Mr Lawrence was knifed in the chest after he went to help one of his pupils last December.
The boy was ambushed and attacked in revenge after a "trivial" incident of barging into the corridor with an older boy who "boasted he was a Triad member" and threatened to bring his friends to beat him up, the prosecution alleged.
On the day of the attack, with Mr Lawrence and his pupils watching, the armed gang of 12, some wearing a uniform of dark clothes and bandanas, arrived and carried out a pre-planned "military operation".
One gang member bludgeoned the 13-year-old boy's head with a metal pole from behind. As Mr Lawrence "peaceably" confronted their leader to find out what was going on, he was stabbed.
Mr Bevan said: "A few grave crimes each year command public attention beyond the norm and cause us collectively to pause and reflect.
"The murder of Philip Lawrence shortly before Christmas last year was one such.
"A man who in life was unknown beyond his own wide circle of family, friends and work environment was in death thrust into the national spotlight.
"The reasons include the nature of the man, the tragic waste of life which his death represents and the nature of the crime which brought it about."
As Mr Lawrence's widow, Frances, listened, Mr Bevan said: "It was a tragic waste of the life of a genuinely good man for the worst reasons."
Mr Bevan said: "Part of his job involved looking after the safety of his pupils and it was in seeking to protect one of them that he died.
"His attacker is also a leader but of a rather different type.
"He, although only 15 at the time, was old enough to lead a gang of youths in a deliberate, concerted and cowardly attempt to attack one of Philip Lawrence's pupils as that pupil left the school gates.
"The gang was armed and when Philip Lawrence confronted him perfectly peaceably in an attempt to find out what the trouble was, the youth, acting presumably on a mixture of bravado and adrenalin, stab-bed him in the chest."
The alleged killer was from a Filipino background and did not attend Mr Lawrence's school, the jury was told.
Mr Bevan said: "For some time he had been involved in a street gang called Woo Sing Woo, which pretended to be a juvenile equivalent of a Triad gang and consisted of a number of youths, largely of Filipino background, meeting in Camden or the West End of London."
The younger defendant was a pupil at Mr Lawrence's school - St George's Roman Catholic School in Maida Vale - and had plotted with fellow gang- members to attack the boy in the year below him after their "trivial" argument, the prosecution alleged.
He said one 15-year-old "reluctant" gang member had showed the courage to break ranks from the "Triad hopefuls" since the incident to tell police what had happened.
He was encouraged to take a day off school as there would be a fight. "It'll be a laugh," he was informed by one gang member.
The gang was then divided into three teams - one to lead the fight, the second to help out and the third to be "careful of attack from behind".
At all times, the youth accused of murdering Mr Lawrence was in charge of the planning, the jury heard. When told the school's headmaster would be at the front gates, the gang leader said not to worry about him, it was alleged.
The trial was adjourned until tomorrow.Reuse content