Images of between 50 and 60 students at a private school were found on the computer of a convicted paedophile, the school's executive principal has said.
American national William Vahey, 64, who was found dead last month, taught history and geography to students at Southbank International School in London from 2009 to last year.
The teacher, who is also believed to have abused children at other schools around the world, killed himself two days after investigators filed a warrant to search a computer drive belonging to him containing pornographic images of at least 90 boys aged from 12 to 14, who appeared to be drugged and unconscious.
The school's chair of governors, Sir Chris Woodhead, said he has been left sickened by the revelations, describing the matter as the "worst thing that I've ever been involved in in 40 years of education".
The investigation is being led by the FBI, which said the photos were catalogued with dates and locations that corresponded with overnight trips that Vahey had taken with students since 2008.
Vahey was convicted over a child sex offence in California in 1969 but appears to have failed to sign the sexual offenders' register, which allowed him to slip through the net and go on to work in several schools.
In a letter to parents, Graham Lacey, e xecutive principal at Southbank International School, urged any pupils who had experienced anything disturbing to come forward.
"This disclosure which we learned this morning, left us all appalled," he wrote. "As a staff we feel upset, angry and betrayed. We can only imagine what you as parents must feel."
The letter, which was sent out this afternoon, adds details of the checks that were made when Vahey joined the school, when he was subject to a Criminal Record Bureau (CRB) check, which is standard procedure and revealed nothing untoward.
A second check, three years later, produced similarly positive results and Mr Lacey added that three references the school obtained were "outstanding" and " specifically stated that there were no safeguarding issues".
Mr Lacey said he was an " extremely popular teacher" and several parents had even expressed their sadness about his leaving the school.
He said there had been one incident that occurred, which wa s investigated by school management at the time, but b oth the parents and the child stated they did not want to pursue the matter any further.
He added that he has subsequently been told by some parents that " rumours were in fact circulating amongst students and parents" but said they did not reach the school's management. "T here are perhaps lessons here to be learnt," he admitted.
Pupils are being offered counselling and a meeting for parents is due to be held next week, which will also be attended by the police and Westminster child protection agencies.
Former Ofsted chief inspector Sir Chris said: "Everyone at the school is deeply shocked by what we heard on Tuesday.
"Our two priorities now are to communicate as much information as we have as quickly as we can, and to help the police as much as we can in what is now an international police inquiry into the activities of this man."
He added that Vahey had an "immaculate record" and had never given any staff at the school any cause for concern. "He was a very popular man, both with staff and students," he said. "He has managed to deceive his colleagues in schools all around the world for 30 years."
Earlier, he told Sky News he felt physically sick when he heard the news, adding: "This is the worst thing that I've ever been involved in in 40 years of education."
Vahey taught in various international schools, which has led the FBI to believe his victims are from all over the world. Because of the fact that he drugged them, s ome might not even know they were abused.
He was confronted about the images by a colleague at the American Nicaraguan School in Managua, Nicaragua, where he had most recently been teaching, and he confessed that he was molested as a child and had preyed on boys all his life, plying them with sleeping pills before abusing them.
The FBI's special agent Patrick Fransen said: "I've never seen another case where an individual may have molested this many children over such a long period of time.
"I'm concerned that he may have preyed on many other students prior to 2008."
Vahey, who had a home in London as well as in South Carolina, was found dead in Luverne, Minnesota, on March 21.
Scotland Yard said it was helping the FBI with its inquiries.
A spokesman said: "Officers from the sexual offences, exploitation and child abuse investigation team are assessing and evaluating intelligence passed to the MPS (Metropolitan Police Service) by US authorities, and actively seeking any evidence whilst working with partner agencies to ensure that potential victims are supported."
Scotland Yard said its Sexual Offences, Exploitation and Child Abuse Command are carrying out an investigation to identify and help any victims.
Detectives are working c losely with the FBI, the National Society for the Prevention of Cruelty to Children and the Southbank International School to ensure victims get help.
P olice believe the alleged victims are boys, who may be aged between 10 to 14 years.
Scotland Yard said officers searched a residential address in north London on April 22 at the request of the FBI. No one was present at the property but a number of items were seized and are being examined, a spokesman said.
A Department for Education spokesman said: "Ministers are being kept in close touch with the ongoing investigations and will make decisions on any intervention in due course."