Fiona Camm, from Copthill school, near Stamford, Lincolnshire, wrote her winning piece, Metamorphosis, about her pet Benny, who came into her life on September 12.
She is one of the six pupils - three primary and three secondary - whose work was chosen for publication in today's Friday magazine.
Nearly 10,000 children, aged seven to 14, entered the autobiographical writing competition, now in its fifth year, spurred on by free materials provided in The TES and in TES Primary magazine.
There were 22 finalists who celebrated their success at Shakespeare's Globe yesterday in the company of parents and teachers.
They received pound;100 each, plus pound;400 to pay for a writer to work in their school, with prizes funded by McDonald's. The six overall winners also won a laptop computer for their school.
The two other primary winners are Esme Jones, nine, of Landscore primary, in Crediton, Devon, and Amelia Lester-Hinchliffe, 11, from Castle Cary primary, Somerset.
The secondary prizewinners are Jennifer Williams, 13, from Weatherhead girls' high, in Wallasey, Merseyside; Sarah Fish, 13, of Haberdashers'
Aske's, Elstree, Hertfordshire, and Anna Bucks, 14, of James Allen's girls'
school, in East Dulwich, London.
The six overall winners were chosen by poet Michael Rosen and Whitbread-winning novelist Jamila Gavin. Ms Gavin said: "All these young writers showed a marvellous ability to turn their feelings and memories into creative writing that communicated with the reader."
Mr Rosen praised entries in the special needs category, introduced this year. He said: "I was especially touched by these. We need more voices like them".
The names of all the finalists are listed in today's Friday magazine and the three junior winners will have their work published in the JulyAugust Primary magazine.
Friday magazine, 8