Tyrell Henry was walking to school Wednesday when he was attacked by the dog, the Chicago Tribune reported.
"When it first bit me, when I was on the ground, I started kicking it," said Henry, a sophomore at Urban Prep Academy. "Then I hit it with my book bag."
Henry said the dog was still attacking him when a Chicago police sergeant arrived.
The officer first tried to scare the dog away by yelling at it, but then fired at it. The pit bull then ran toward the sergeant, who fired again, killing the dog.
"Thank God for that officer," Henry, who received 19 stitches at a local hospital, told police later. "That dog would have killed me."
Police said that the dog was involved in the attack of a 63-year-old man earlier Wednesday. That man was expected to receive stitches for his injuries, said Officer John Mirabelli.
There was no report on the dog's owner.