Once Upon an Ordinary School Day by Colin McNaughton, illustrated by Satoshi Kitamura

From chapters.ca:
It was an ordinary morning when the boy woke up, and it was an ordinary journey to school. But when Mr. Gee bounced into the classroom, everything changed. "He's bonkers," the children whisper. "What's he on about?" But Mr. Gee plays rousing music which thunders around the classroom, and asks what it makes them think of. Stampeding horses! Racing Cars! Elephants! "Write a story about it," laughs Mr. Gee. And the ordinary boy began to write, and the words tumbled out of him pell- mell. It was extraordinary!