BATON ROUGE - East Baton Rouge school officials say they're investigating after a young student was left on a school bus Tuesday.

The boy's mother, Ashley Williams, said she was shocked to get a phone call from a business on Florida Boulevard saying they had her son.

"Why is he at a furniture store? Why isn't he at school?" Williams said, recalling her conversation with a worker.

The 4-year-old, a Southdowns Elementary student, was picked up by the bus around 8 a.m. Tuesday. The EBR Parish School System says the boy was left on the bus at the end of the route. 

According to the sheriff's office, the child was found onboard the bus, left in a parking lot. Authorities were alerted after a passerby heard the child's calls for help.

"He said he heard screaming and hollering, and he went to the bus."

Williams said the good Samaritan, an employee at the store, couldn't get in because the vehicle was locked up. Instead, he had the boy open one of the windows, allowing him to climb on the side of the bus and pull the child out.

When she finally made it over to the business, Williams said her son was drenched in sweat. It was apparent he had been in the bus for some time.

"How she left my child on that bus, I don't know what she was thinking... He was on that bus by himself for almost two to three and a half hours," Williams said. "No air circulation... Thank god it was not as hot today as it has been in the past."

The boy was treated for heat exhaustion but is expected to be OK.

The school system says bus drivers are required to check the vehicles at the end of their routes each day, and it is now investigating what happened.

The driver involved has been fired.