A San Diego mother whose disabled son was left alone on a sweltering school bus says the principal of the school tried to cover up the mistake.

Ena Walters says Monday morning, her 11-year-old son was supposed to be dropped off at summer school at Walt Whitman Elementary School in Clairemont.

"My son was found wandering the transportation yard," she told 10News.

Walters says not only is she upset that it happened, but upset because she believes the principal covered up what actually happened.

"That the driver turned around and brought him back to school," said Walters, referring to what she says the principal told her Monday.

But that is not what the boy or the district told 10News.

"It just wasn't adding up with what my son was saying," she said.

Walters says her son was supposed to be at school by 8:55 a.m., but at about 9:30 a.m., another driver saw the boy standing near another bus in the school yard in Kearny Mesa.

The mother says her son woke up because of the heat – which was roughly 81 degrees that day – but much warmer inside a hot vehicle.

"But I do know that if I left my son in the car that, you know, I would have been arrested," she said.

In May, Zia Estrada, another special needs student who was left alone on her San Diego Unified school bus.

It was one of the reasons Walters says she decided to contact 10News. She said she wanted to let us know that it happened again.

Principal Olga Venegas would not talk with 10News on camera, but San Diego Unified School District spokeswoman Linda Zintz told 10News:

"We are taking this very seriously. In the future, we will implement additional training for bus drivers to make sure they understand and follow protocol."

Walters wants the driver and the principal to learn from their mistakes.

"It's protocol," she said. "Just check the seats for the kids."

10News was told the bus driver has been put on unpaid leave while they investigate. Part of that investigation will determine if she walked down the aisle and checked the bus before she left it in the yard.