A freak accident that left schoolchildren rattled could have been a lot worse when a Philadelphia school bus lost its two back tires en route to school. "She sent me a picture of a tire on the ground, and I said, 'What is that?' and she says, 'That's our schools bus tires. They both fell off,'" said one mother whose daughter shared photos taken of the incident. Another bus was called, transporting the students safely to school.Parents now want answers from the school district about how this could happen. The district is investigating and questioning the third party contractor. However, the district says a bus losing its tires has never occurred before.