A California bus driver has been jailed for two years after the death of a 19-year-old autistic student who was found unresponsive on a bus he was driving in the middle of a heatwave.

Armando Able Ramirez, 37, left Hun Joon Lee on the bus after picking him up to take him to Sierra Vista Adult School at 8am in September 2015.

It was only when he didn't return home after school at 3.30pm that a search began.

He was found dead on the floor of the school bus that had collected him earlier that morning. The windows were all closed; temperatures in the area that day had reached 96 degrees.

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Armando Abel Ramirez, 37, of Rialto, pleaded guilty to one felony count of dependent adult abuse resulting in the death of Hun Joon Lee

Hun Joon Lee (pictured with his mother), a 19-year-old who had autism and the mental capacity of a three year-old, died after being found unresponsive on a school bus

Bus driver Armando Ramirez, 37, has now been jailed for two years over Lee's death

Ramirez, of Rialto, California pleaded guilty to one felony count of dependent adult abuse resulting in the death of Hun Joon Lee.

It was revealed in court that Ramirez was a substitute driver for Lee’s bus and was working a split shift on the day of Lee’s death, prosecutors said.

Lee, a Sierra Adult School student, was found laying on the floor of the parked bus in September 2015

He told authorities he believed Lee had got off the bus to go into the school in the morning, however he did not walk to the back of the bus or even look over his shoulder to make sure no one was left in the bus before the end of his morning shift.

It was only when he returned hours later for the second part of his shift and had been told that Lee was missing he was discovered.

Lee's mother became concerned after not seeing her son return home from school at his usual time of 3.30pm.

She called the school to ask about her son's well being.

The school, however, told her that her son had never arrived in the morning.

Eun Ha Lee rushed to the school to look for her son, calling police on the way.

As she, police and school and bus personnel searched for the teenager, her son was found on the bus.

Paramedics arrived to find drivers attempting to administer CPR on Lee, but he was unresponsive and efforts to revive him were unsuccessful.

Lee's mother, Eun Ha Lee, said her son got on the bus at 8am and never returned home after school at his usual time of 3.30pm

The teen was found unresponsive on the bus. Attempts to revive him were unsuccessful .Temperatures in the area hit 90 degrees while he was on the bus

Lee was discovered in the aisle of the bus. His mother explained that the teen was nonverbal and wouldn't have been able to call out for help.

At the time of his death, Eun Ha Lee said that her son's death was devastating.

'My boy is a very, very precious boy,' she said to KTLA. 'I don't know other people, how they think about my son, but my son is perfect to me.'

She said she didn't understand how her son was left behind because there were only three students on the bus that day.

'I feel like, we are nothing,' she said. 'They killed my son. Technically, they killed my son.'