THE BRONX - Police are searching for a driver who they say fled the scene after crashing his minivan into the back of a school bus carrying two students in Parkchester this morning.The driver of the bus says he heard the white minivan hit another car up the block along Metropolitan Avenue, but by the time he looked in his mirror, he says the out-of-control vehicle was already underneath the bus, lifting the back wheels off the ground.Eyewitnesses say they were shocked to see the driver of the minivan limp away after the accident without checking on the kids aboard the bus. Neither of the students was injured in the accident. Parents say they were lucky the driver had just started his morning pick-up and the bus wasn't yet filled with kids.