A Toronto mother's world turned temporarily upside down when it appeared her young son had gone missing after boarding a school bus.

"Oh boy. I started breaking down into pieces," Veronica Wells told reporters Tuesday after being told her son Bradley, 4, had gone missing.

"My heart starts pounding and my head starts spinning and I began to worry a lot ... It's a parent's worst nightmare."

However, the nightmare only lasted 30 minutes. Police found Bradley aboard the bus, which the boy only started taking seven days ago.

"In my whole entire life, I never thought I would experience something like that," Wells said.

The four-year-old boy was leaving junior kindergarten at St. Martin de Porres elementary school, located at 230 Morningside Ave., en route to his daycare. The bus didn't stop at its usual stop at its usual time -- 11:10 a.m. -- or place.

Gretchen Schuler, principal of the school, told CTV Toronto: "She (Bradley's teacher) was absolutely sure she put him on the bus. She was correct. She had put him on the bus. And the information we got from the bus company was that he had not been on the bus."

The bus company told Veronica Wells the same thing.

Police in 43 Division were called. They began an urgent search, but the bus's driver said found the youngster on the bus under a seat about two kilometres away from his stop.

"Complacency may sometimes set in. I'm not saying that happened here, but a child was able to get under a seat," Sgt. Doug Brown told reporters.

Veronica said bus drivers have to be careful, especially when dealing with children under the age of five.

The general manager of the bus company told CTV Toronto that the driver is under review and that the company has a zero-tolerance policy for such errors.

In addition, it has a child-check policy that requires drivers to check the bus at the end of their route. But they have no idea why the bus didn't stop in this instance, he said.

With a report from CTV Toronto's Dana Levenson