Mrs Sibbold, now 40, set the exercise 14 years ago at the Priory School, in Dorset, where she still teaches. "The children all wrote to their favourite authors, so several chose Roald Dahl," she said. "He was definitely the most popular, but we weren't sure if we would get any replies back."
Dahl, who died in 1990, wrote back enclosing a 10-line untitled poem, depicting a typically unpleasant character. When Mrs Sibbold read out the description of Mr Unsworth, the sadistic history teacher, the impact was immediate. "Children think the poem is hilarious," she said. "They burst out laughing. We were just so delighted he'd written back, the value of the letter didn't occur to us. So I took it home and kept it on the shelf."
Fourteen years later, teachers were asked to select their favourite verse for national poetry day, and she recalled the poem she had at home. "I was genuinely surprised when the head said he couldn't find it published anywhere," she said. "We're thinking of framing it and putting it up in school."
Dahl has written extensively about his unhappy years at St Peter's boarding school in Weston-super-Mare. His autobiography, Boy, is filled with tales of sadistic teachers and his novel Matilda is peopled with gruesomely overbearing teachers.
Today's Priory pupils like the poem too. Ten-year-old Scott Coulidge said:
"This was a lot better than some of the other poems he's written. It's more humorous, and shorter."
Thomas Jarvis, 10, said: "It must be quite valuable: now that he's dead, he can't write poems for any more schools."
A comprehensive archive of Roald Dahl's work will be available at the Roald Dahl Museum and Story Centre, due to open in 2005. www.roalddahl.com