BOSTON, Ma. -

A charter bus carrying high school students from Pennsylvania crashed Saturday night when it attempted to navigate under a low bridge in Boston.

More than 30 people were injured, four of them seriously in the accident that left some trapped for more than an hour.

The bus, which was carrying 42 students and adult chaperones, was heading back to the Philadelphia area after visiting visit Harvard University when it struck an overpass on Soldier's Field Road.

Authorities said the Calvary Coach bus did not belong on the road where a 10-foot height limit is in place and over-sized vehicles are not authorized.

State police say they will continue to investigate the crash.