Parents at the elite Southbank International school in London were told on Thursday evening that a paedophile American teacher held images of dozens of their children on his computer, some of which showed they had been molested.

Graham Lacey, the school's head, said that 50 to 60 images had been found on a computer belonging to William Vahey, plunging hundreds of families into an agonising wait to discover if their children are among those pictured.

Vahey, who taught at the school between 2009 and 2013, killed himself in Minnesota last month, aged 64, after being found at an international school in Nicaragua with 90 images dating from 2008 of boys believed to have been abused on school trips.

The FBI, which has been leading a global investigation, believe he drugged his victims before molesting them so they almost certainly do not know whether they were attacked. When Vahey taught history and geography at the London school, he led several trips abroad.

"This disclosure, which we learned this morning, left us all appalled," said Lacey in a letter to parents. "As a staff we feel upset, angry and betrayed. We can only imagine what you as parents must feel."

The school's investigation is focusing on school trips Vahey led to places which parents and former pupils said included Jordan, Venezuela and Nepal. Lists of participating pupils have been passed to Scotland Yard and parents have been asked to come forward if children "recall any disturbing incident".

The school's chair of governors, Sir Chris Woodhead, a former chief inspector of schools, said the unfolding crisis was "the worst thing that I've ever been involved in 40 years of education".

Outside the school on Portland Place in Westminster on Thursday, parents were struggling to come to terms with the abuse crisis and said they felt "upset and betrayed".

Speaking outside Southbank, some wondered how to handle the news that their children may have been molested even if they did not know about it at the time because they were drugged.

"Even if they find my son in the photos, I would rather not know and just forget about it," said the mother of a child who was in Vahey's class and had gone with him on a field trip.

Many children had suffered sleepless nights since news of Vahey's abuse began circulating, after the FBI issued an appeal for more information on Tuesday. "He was an incredibly loved teacher and so the kids are feeling conflicted," one mother said. "They were mourning him a few weeks ago. It is horrific; there couldn't be anything worse than what the kids are going through." She added, they are "scared and distraught, can't believe it's true, don't want it to be true".

There was also frustration among parents at the lack of information from the school. Children were called into a special assembly on Thursday morning, but a meeting for parents will not take place until next Monday. "Every teenage kid who went on those trips is asking: 'Was it me?'" said one mother. "Chances are that some of our kids have been victimised, but we don't know anything."

Lacey, who runs a school used by many international business people and diplomats, also admitted that concerns had been raised about Vahey's behaviour while he was working there as a geography and history teacher.

He said there had been one incident relating to Vahey's relationship with children, which was investigated by school management at the time, but both the parents and the child stated they did not want to pursue the matter any further.

The headteacher 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. "There are perhaps lessons here to be learned," he said.

It has now emerged Vahey was jailed for child molestation in California in 1969, but that conviction was not picked up by the school's vetting procedures, which the school insists were extensive and went back over 17 years of his career.

However, the Guardian has established that senior staff raised the alarm about vetting processes for new teachers three years ago, shortly after Vahey was hired. Woodhead said members of the school's management board complained to him about the "competence and ability" of a staff member with responsibility for vetting. The staff member remains in post, after Woodhead said he found the complaints to be unfounded.

In 2010, a regulatory report by the Independent Schools Inspectorate warned the school over its hiring procedures – and specifically, according to Woodhead, raised concerns that Criminal Records Bureau (CRB) checks were not kept up to date. The inspectors wrote in their report: "Some aspects of staff recruitment have not been completed properly."

But the school said it had carried out CRB checks on Vahey and took three references, including two from his previous school in Venezuela and one from a school in Jakarta. He had taught in international schools in eight countries. His CV showed he had been registered as a teacher in the state of New Jersey in 1986, and Woodhead said it was reasonable to have assumed that would not have been the case if he had been convicted of child molestation.

The school said it has recruited professional counselling services from outside the school to support pupils and staff. "Our immediate priority is to support both students and parents at this very difficult time," said Lacey.

Vahey's wife, Jean, is a prominent figure in the international schools movement and is executive director of the European Council of International Schools in London. She was superintendent of the Escuela Campo Alegre in Venezuela at the same time as Vahey taught there between 2002 and 2009. The ECIS said on Thursday she was on compassionate leave. There is no suggestion she was involved in any wrongdoing.