An evening out in London for 62-year-old civil servant Ian Baynham and a friend ended horrifically in Trafalgar Square when he was attacked, stamped on and kicked by drunken teenagers screaming homophobic abuse. The incident led to a former private-school pupil and her ex-boyfriend being convicted of manslaughter at the Old Bailey today.

Baynham, who was gay, was pushed to the ground and then stamped on and kicked as he lay unconscious during the attack in September last year. He died in the Royal London hospital from brain injuries 18 days later without regaining consciousness.

The court was told that in a scene likened to Stanley Kubrick's film A Clockwork Orange, Ruby Thomas, who was then 17, smiled as she kicked Baynham after he had been knocked down by another teenager, Joel Alexander, while tourists looked on in horror. Police later found Baynham's blood on her handbag and the ballet pumps she had been wearing.

A former boyfriend of Thomas, now 18, had told the jury she was not the type of girl to have carried out such an assault, but she had a previous record of violence, having been convicted of assaulting a bus driver in Northumberland Avenue, near Trafalgar Square, when she was 15.

Thomas is a former pupil of Sydenham high school for girls in south London, where day fees are currently more than £12,000 a year. The school says on its website: "We know our girls and they know us … their school is very proud of them." The Good Schools Guide states that pastoral care is "supportive and caring … but not cosy … girls are presented with opportunities; they must grab them and run".

Alexander, now 20, of Thornton Heath, south London, was convicted with her. A third defendant, Rachael Burke, 18, of Upper Norwood, south London, was convicted of affray at an earlier trial. They have been remanded in custody and will be sentenced in the new year.

The court was told that Thomas, of Anerley, south-east London, had been acting in a "lairy, mouthy way" and flirting with random passersby before turning on Baynham and his friend Philip Brown and screaming "fucking faggots" at them as they crossed the square.

When Baynham confronted her, there was a scuffle during which she hit him with her handbag. Alexander knocked him to the ground, causing a severe brain injury as his head hit the pavement.

Brian Altman QC, prosecuting, told the court: "That did not suffice. There is evidence that the female defendants then began putting the boot into Mr Baynham, who was still prone on his back, clearly unconscious and in distress … shocked onlookers saw repeated stamping to his chest and forceful kicks to his head." He said the girls had been fuelled by copious amounts of alcohol.

After the conviction Baynham's sister Jenny spoke of her distress at learning of the homophobic abuse suffered by her brother, who was a team leader in the Home Office border control office.

She said: "My brother was an ordinary, honest, decent man, loved by his family, especially our mother, and his many friends. His only crime seems to have been to stand up for who he was and it is impossible to make sense of the dreadful event that led to his death. I am not normally angry but I just feel it is so wrong.

" It should never be. We should not be able to treat each other like that. It doesn't matter who you are, we should be able to give that to everyone, whoever we come across. I thought that we had moved on a long way and I am really surprised that this sort of thing is still thrown up and is still an issue.

"It is the general public's responsibility to do something about that and report more of these sorts of incidents. There is still a huge prejudice relating to homophobia … a lot more needs to be done. Everybody has certain feelings about different sections of society but it is about tolerance, that is important."

Detective Inspector Paul Barran, who led the police investigation, said: "Our investigation clearly showed a background of aggressive, drunken behaviour that led to hostile confrontations with others – decency and respect were nonexistent."

During the trial the prosecution rejected an application for Thomas to move from a Midlands bail hostel to return to the capital, adding: "London and this woman don't go together."

George Richardson, a close friend of Baynham, said: "In the case of Ian and Philip you are talking about two perfectly ordinarily dressed, ordinary men, doing nothing to attract this kind of abuse."

He described his friend as "a perfectly normal man who just happened to be gay … Ian's death resulted from an attack motivated by homophobia. Mindless violence generated by a total absence of tolerance and fuelled as we now so often see by alcohol, has pointlessly robbed myself and others of an excellent friend. Ian died because he was proud and honest about who he was."

Ben Summerskill, of gay pressure group Stonewall, said: "The perpetrators of this crime were educated in Britain's education system within the last five years, demonstrating how much more needs to be done to tackle homophobia in our schools before it festers into violence on the streets."

Kathryn Pullen, headteacher of Sydenham high school, said tonight: "We are a diverse community that promotes tolerance and individuality. We expect the highest standards of behaviour from our girls at all times."