Prosecutors say 37-year-old Armando Ramirez did not check to see if any students were still on the bus at the end of his shift Sept. 11, 2015.
Paul Lee, 19, who had severe autism, was found dead inside the hot bus hours later.
All of the windows on the bus were closed, and the temperature that day was near 90 degrees, prosecutors said.
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Lee was a student at Sierra Adult School who could not verbally communicate and needed special care, officials said.
Ramirez was a substitute driver for Lee's bus that day and was working a split shift, authorities said.
According to prosecutors, Ramirez did not check the bus after ending his first shift in the morning and returned to the bus yard to fill out paperwork before heading home. Ramirez says he thought Lee had gotten off the bus in the morning.
When he returned for his second shift of the day, prosecutors said Ramirez was notified Lee was missing.
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That's when prosecutors said Ramirez checked the bus where he found Lee unresponsive and called for help.
Ramirez faces a felony charge of dependent adult abuse resulting in the young man's death.
He is currently out on bond. If convicted as charged, Ramirez faces a maximum of nine years in state prison.
Ramirez is expected to be back in court for another hearing May 27.
A new law has recently been proposed to ensure bus safety. The Paul Lee Bus Safety Law, proposed by Sen. Tony Mendoza, calls for every school bus in the state to have child safety alarm systems in place. All 1,300 Los Angeles Unified School District buses, as well as 600 contracted buses with the district, already have them.
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The bill was unanimously approved in early April by the California Senate Transportation and Housing Committee. The bill must go through two more committees before it heads to the entire Senate.
The alarm works to keep bus drivers more active in checking the vehicle. As soon as the bus driver turns off the ignition and headlights of the vehicle, he or she has 15 to 20 seconds to walk to the back of the bus
If approved, the bill would start in January 2017.