"There was no doubt a big problem with me, but what it was, no one was sure then," says Gardner, who was born in Birmingham, England.
Then, she was diagnosed as "dyslexic", which means she couldn't learn to spell words properly. But that hasn't stopped her from turning into an award-winning children's writer.
Yet success didn't come easy.
"When my teacher told me I was 'unteachable', I hoped it meant I didn't have to go to school anymore. I thought, 'If I was not teachable, then what's the point of me being there?'"
She had her reasons for not liking school.
"I was badly bullied all through my early education. Because no one knew what was wrong with me, I was always a bit of an outsider," she says.
Some teachers were not exactly helpful, either.
"There was this terrifying teacher who my parents paid extra money to teach me outside the classroom. She gave me intelligence tests that I couldn't do. She made it quite clear that I wasn't bright," Gardner says. "And she tried to 'cure' me by teaching me phonics. When she was shouting 'coat, moat, goat', I'd be sitting and longing for her phone to ring, then she'd pick it up and say, 'I can't talk now, I am with a patient', while she carried on talking."
She was transferred to another school - the School for Maladjusted Children.
"Dr Bullen, who ran the school, was inspirational; she didn't patronise children," the author recalls. "She gave me lots of confidence and never treated me as if I was stupid."
Gradually she learned to read and write. And when she read a novel, Wuthering Heights, for the very first time, she was thrilled. She couldn't stop reading.
"I was 14 then. It was an absolutely amazing experience," she says.
"The whole room fell away. The noise, the other children were all gone. Suddenly it was just me and Heathcliff [the lead character] on the moors ... It was a turning point in my life."
She went on to earn a degree in theatre at Newcastle University, where she worked as a designer. After 15 years in theatre, she turned to writing. "I assumed my dyslexia would be a true disability, but it turned out to be the start of something amazing," she says.
Her first full-length novel, I, Coriander, was published in 2005. The book won the UK's Nestle Children's Book Prize Gold Award.
Gardner says she has put some of her personal experiences into her characters. In her latest book Maggot Moon, which was published last month, the main character Standish Treadwell is severely dyslexic. Yet, his vision and courage serve him well in a quest to track down his missing best friend, Hector.
Having dyslexia wasn't all a bad thing, the author says. "There're many wonderful things about dyslexia," she explains.
"We are very visual. I can see scenes in my books in three dimensions in my head. I can walk around [my settings] as if they were real."
In a way, the condition has nurtured her imagination, she says. "Dyslexia is me. It's not a disease; it's a way of thinking. There is no way I can 'extract' dyslexia from who I am."
To youngsters who want to write, Gardner has this to say: "[You should not] be put off by anyone telling you that you can't do something - believe in your dreams."
Sally Gardner will be present at this year's Hong Kong International Literary Festival, which opens on October 4. For details, go to www.festival.org.hk