The best-selling author of Eats Shoots Leaves, says: "There just isn't enough emphasis on writing in the school system. Not enough correcting of errors; not enough enthusing about the mechanics of language."
Ms Truss has written a 16-page illustrated version of her book, Eats, Shoots Leaves: why, commas really do make a difference! for primary pupils.
She says: "I think getting children to notice punctuation is a big moment in their reading."
At pound;23,625-a-year Harrow school, sixth-formers are being given remedial spelling lessons after one in eight failed a basic literacy test.
Many pupils with GCSE A-grades could not spell simple words or punctuate simple sentences.
English teacher Tom Wickson said: "Technical accuracy is not prized as highly as it once was."