An elite London school has become embroiled in an FBI-led international child abuse investigation into a highly regarded teacher who is believed to have molested more than 90 boys.

William Vahey, a 64-year old American who taught at the Southbank International School in Westminster between 2009 and 2013, killed himself in Minnesota in March after confessing to drugging and molesting children while on field trips.

He had been found with a memory stick that contained sexually graphic images of at least 90 boys who appeared to have been drugged. They were catalogued with dates and locations that corresponded to his overnight field trips with students beginning in 2008 when he was teaching in the Escuela Campo Alegre in Caracas, Venezuela.

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Southbank's chair of governors, the former chief inspector of schools, Sir Chris Woodhead, said the school was "deeply, deeply shocked" by the FBI's announcement of the suspected mass crime. The fee-paying school is used by foreign executives and diplomats. Vahey taught boys aged 11 to 16.

Woodhead confirmed Vahey took part in several field trips while at Southbank and said parents at the school were last night showing "considerable concern".

The school is planning to either hold a meeting or will write to parents within 24 hours about the crisis. Families are also to be offered psychological counselling.

The FBI said that Vahey was jailed in California in 1969 for child molestation, but Woodhead said that nothing showed up on criminal record checks or references taken in the UK and abroad by the school, where Vahey had taught history and geography.

Scotland Yard is helping assess the FBI's intelligence about "potential alleged offences against children in London". The FBI said it was trying to identify the dozens of children pictured in the photos.

Patrick Fransen, an FBI special agent, said: "He had access to children because of his position of trust. The manner in which he committed these acts – while the boys were unconscious – may have inhibited them from knowing what happened, making it impossible for them to come forward at the time of the molestation."

As well as teaching in London, Vahey also worked in schools in Caracas in Venezuela, Jakarta in Indonesia, Dhahran in Saudi Arabia, Tehran and Ahwaz in Iran, Athens, Madrid and Beirut, during his career in education. The FBI has urged anyone who believes they may have been a victim to come forward.

Vahey was working in Nicaragua when he was exposed in March by his employer who discovered the digitised images, the FBI said. When confronted about the images by a school administrator, he confessed he was molested as a child and had preyed on boys his entire life, giving them sleeping pills prior to the molestation. Within days he killed himself.

"This is a completely shocking revelation," said Woodhead. "We are co-operating with what is now an international police inquiry. We are supplying the police with all the information they want; trips Vahey took and so on.

"When he was appointed we ran a criminal records bureau check and again three years later. In each case there was no cause for alarm. "We took three references [from previous schools], two from Venezuela and one from Jakarta. All of the references were very positive and said he was an outstanding member of staff. They said he was very willing to take school trips but with what we know now that is not as positive as it was when we read them."