A 54-year-old former geography teacher from Hertfordshire who photographed a colleague molesting a teenage boy has been jailed for 20 months.
John Foxley from Borehamwood admitted a string of sex offences involving children, dating back to the late 70s.
Middlesex Guildhall Crown Court heard Foxley used his position of trust to abuse boys and take indecent pictures.
Police have called Foxley a prolific paedophile and say there may be more victims who have not yet come forward.
Foxley admitted aiding and abetting the indecent assault of a child who he photographed being abused by Shaun Tyler.
Tyler, 37, an American teacher who Foxley met when both men taught at an East Sussex school in the 1980s, was jailed for nine months last year for abusing the 14-year-old boy.