BOSTON — A charter bus carrying high school students from Pennsylvania crashed when it attempted to pass under a low bridge in Boston on Saturday night, injuring more than 30 people, four seriously, and leaving some trapped for more than an hour, authorities said.

The students had been in the area to visit Harvard University. The bus, which was carrying 42 students and adult chaperones, was heading back to the Philadelphia area when it struck an overpass on Soldier’s Field Road, a major crosstown road, around 7:30 p.m., Massachusetts State Police said.

Authorities said the Calvary Coach bus did not belong on that road, which was not authorized for over-sized vehicles. State Police were investigating whether charges were warranted against the driver, who apparently failed to see a sign prohibiting the vehicle from taking that route.

Ray Talmedge, owner of the Philadelphia-based Calvary Coach Bus company, said his driver was being interviewed by police. Talmedge said the driver, who also drives a school bus, didn’t know about the road restrictions.

The students were part of a Destined for a Dream Foundation group, Talmedge said. Officials with the Bristol, Pa.-based group, a nonprofit that helps underprivileged youth, refused to comment on the crash when reached by phone.

The group’s Facebook page said the trip to Harvard was to “visit the campus, sit with the office of cultural advancement, followed by a tour of the campus … followed by Harvard Square (shopping, eating, site seeing…etc…) This should be a fun time for all!”

The bus suffered significant damage in the crash. The front part of the roof was pushed in while the center section bowed downward. Photos posted on the Fire Department’s website showed firefighters standing on the top of the bus using boards to extract people. The last victim was freed from the bus around 9 p.m., according to the department.

The Boston Emergency Medical Services said 34 people were injured, including one who was hospitalized with life-threatening injuries and three with serious injuries.

The Massachusetts Bay Transportation Authority sent buses to pick up other passengers and get them out of the frigid temperatures.

Soldiers Field Road curves along the Charles River and passes by Harvard and Boston University. It is a major roadway to the Massachusetts Turnpike.