Peter Harvey, 50, attacked the 14-year-old at All Saints' Roman Catholic School in Mansfield, Nottinghamshire, in July last year.
Transcripts of an interview between Harvey and detectives were read out at Nottingham Crown Court.
In it, Harvey said: "I can't remember it too well but I do recall it was like watching it on television, like it was not actually happening to me.
"I can remember the boy saying 'f*** off' and when that happened I was not really there."
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