Description:A high school field trip in Boston ended in chaos when the school’s charter bus hit a bridge.
35 students and their chaperons were injured in the crash on the way back from Harvard University.
Authorities say the driver did not yield to a low-clearance warning sign causing the bus to crash.
Four people suffered serious injuries, one of which is life-threatening
It took more than an hour for rescue crews to free some of the passengers.(0:43)