Authorities said that the 11-year-old Winslow Elementary School student awoke at around 11:00 a.m. Another bus driver spotted her.
Gardener apparently did not know that the girl was still on the bus despite an alarm system on each school district bus that is designed to prevent such a mistake, officials said.
After shutting off the engine, drivers must go to the back of the bus to turn a switch before a loud alarm sounds.
It forces them to walk by each and every seat, so it is unclear how Gardener followed that procedure and left the bus without seeing the sleeping girl.
School District Communications Coordinator John Sbrana said the alarm, once triggered, stays on.
"That can not be disabled by the driver," said Sbrana. "Someone from the transportation department must come out and disable that alarm."
One fellow bus driver told Action News that Gardener has 17 years on the job and is a conscientious, caring bus driver who now, apparently made one big mistake.
Gardener was suspended without pay pending a civil service hearing and a police investigation.