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Now aged 29 and due to marry Prince William on April 29, Miss Middleton's school torment included having her bed soiled by fellow students, Britain's News of the World newspaper reported on Saturday.
"She hated it, absolutely hated it," school pal Jessica Hay said of Miss Middleton's time at the exclusive Downe House girls' boarding school.
"The girls there were horrible. They used to put faeces in her bed and she was very, very badly bullied.
"She was picked on because she was perfect - well turned out and a lovely person. She was not the type of person to stick up for herself," Ms Hay told the newspaper.
Aged 13 at the height of the bullying, the Middleton family withdrew their daughter from the school mid-term and she transferred to Marlborough where she completed her secondary schooling.
Miss Middleton and her prince have named Beatbullying as one of many charities their wedding guests will be invited to support in lieu of gifts.
Details of Miss Middleton's troubled school life are set to be revealed in a new book, Kate, written by Sean Smith and released on May 2, which tells how the royal fiancee was self-conscious about her height and the condition eczema, from which she suffered as a girl.