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8 February 2017, 15:35
He pleaded guilty to 45 sex offences against children
Retired English teacher Mark Frost, 70, pleaded guilty at the Old Bailey to committing 45 offences against nine children in Thailand between 2009 and 2012.
Frost, formerly known as Andrew Tracey, also admitted having sex with two pupils in Worcestershire over three years in the 1990s.
The court heard on some occasions the abuse happened on school grounds and at other times his adopted son was present.
Judge Mark Lucraft told Frost he was responsible for "the most appalling catalogue of sexual abuse" and it was clear he had an "ongoing obsession" with young boys.
He said: "Your conduct towards each and every one of these victims is horrific and deeply disturbing."
In addition to the 13 life sentences, Frost was handed 10-year terms for other sex offences and five-year terms for indecent images.
He made no reaction as he was sent down.