The driver, Armando Abel Ramirez, 37, pleaded guilty to one count of dependent adult abuse resulting in the death of Hun Joon "Paul" Lee on Sept. 11, 2015, according to the Los Angeles County District Attorney's Office.
Lee was found on the floor of the bus at the end of the hot day, with all the windows closed.
Although Lee was 5 feet 11 inches tall and weighed nearly 300 pounds, his family said he had the mind of a young child and he was unable to speak to call for help.
Ramirez is expected to be sentenced to two years in state prison under the plea deal when sentencing is determined on Jan. 30 at the Norwalk courthouse.
Lee's death also resulted in new state legislation that requires an alarm system for every school bus in California, requiring the driver to walk to the back and press a button before exiting the vehicle.