A Catholic school in Chicago is investigating how a 6-year-old boy was left sleeping on a school bus for more than two hours in subzero temperatures.

St. Malachy School officials say Kinjari Penson fell asleep Thursday on the way to an after-school program at a Chicago church. The first-grader didn't wake up until the other children returned to the bus.

The temperatures had dropped to 11 below zero. The boy complained that his feet hurt from the cold and was treated for hypothermia at a hospital. He has been released.

His parents say they're outraged.

Principal Bridgid Miller says the school is working on new safeguards.

Experts have said they are aware of up to 75 such incidents nationwide every year.