Bruce Garner, 71, who recently pleaded guilty to a slew of sexual offenses. He was arrested last year while driving for the Marple Newtown School District.
A search warrant found more than 100 so-called "upskirt" videos of preteen and teenage girls on his cell phone, according to authorities.
Delaware County District Attorney Jack Stollsteimer said Garner befriended the girls and would have conversations with them.
It was only when a few of the girls noticed he was holding his phone at his side as they got on and off the bus that they told their parents, Stollsteimer said.
The parents immediately told the school district and the investigation began.
Garner is set to be sentenced in June.