A teacher who abused a nine-year-old boy during a PE lesson has been jailed for 12 months at Preston Crown Court.
Anthony Watson, 24, of Pilling, Lancashire, who was also a scout leader, molested the boy at a primary school in Blackpool in June 2004.
Police later found more than 500 pornographic images of children on laptop computers in Watson's home.
After initial denials, Watson admitted sexual assault and nine counts of possessing indecent images of children.
Dennis Watson, prosecuting, said: "He had a sexual fascination with pre-teenage boys which involved him visiting websites which showed pornographic images of pre-teen children, but that was not the extent of his fascination."
Watson joined the school in September 2003 as a geography, IT and PE teacher.
On 15 June 2004, he was leading a PE lesson involving a game of cat and mouse.
He took two boys behind a curtain and, after dismissing one, molested the other child.
The boy later told police he had nightmares about the incident, the prosecution said. Police found a total of 512 pornographic images of children at Watson's home on Lancaster Road.
Sentencing, Judge Philip Sycamore said: "In my judgment all of these offences are so serious that the only possible outcome is an immediate custodial sentence.
"Your opportunistic behaviour resulted in a gross breach of trust when you had knowingly put yourself in a position of working with children."
Watson will have to register with police for 10 years following his release and sign the sex offenders' register. He was disqualified from working with children for life.