About 24 hours after their bus smashed into a Boston overpass, a group of students and chaperones from Bucks County stepped onto another bus to head home.

But at least four of the original 42 passengers remained hospitalized with serious injuries they suffered when the top of their charter bus slammed into the bottom of the overpass.

The students, from a Bucks-area community group, had spent Saturday afternoon touring Harvard University, meeting students, and exploring the historic college town of Cambridge. It was supposed to be just a day trip, organized by the Destined for a Dream Foundation, a Bristol Township nonprofit serving at-risk students ages 5 to 19.

Matt Cruz, a sophomore basketball and track athlete at Neshaminy High School, remained at Boston Medical Center in critical condition. Friends said he underwent two operations and might have serious spinal damage.

Three others remained in serious condition. In all, 35 people were injured. Only seven - including the driver - were unharmed.

The passengers had hardly settled in for the five-hour ride home when disaster struck.

Massachusetts State Police said the bus driver, Samuel J. Jackson, 66, of West Philadelphia, should never have been on Soldiers Field Road. The road is a major crosstown connector to the Massachusetts Turnpike, but it has low clearances and numerous signs warning of a 10-foot height limit.

Jackson was trying to navigate Boston's confusing maze of roads and rotaries, famously challenging to out-of-towners. He looked down at his GPS, then back up and saw the overpass but was too close to avoid hitting it, Ray Talmadge, owner of the Philadelphia bus company Calvary Coach, told WCAU-TV.

Teresa Merrigan of Levittown got a call from her daughter Alana, 17, before rescuers arrived, according to the Associated Press.

"She was screaming and crying and saying that the roof was caving in and that she couldn't see anything, and she hit her head and she hurt her arm," Merrigan said.

Massachusetts State Police said Sunday that Jackson could face traffic citations or more serious charges.

Calvary Coach is registered at 100 N. 58th St. in West Philadelphia. The address belongs to a corner house with no markings or garage, and seemingly no place to park a bus. Neither Jackson nor Talmadge could be reached Sunday.

According to a Federal Motor Carrier Safety Administration database, Calvary Coach hasn't had any recent accidents. But its drivers were cited in June, August, and October 2011 for safety violations, including failure to obey a traffic-control device and not properly logging their time behind the wheel.

State police are looking into whether Jackson had been working longer than allowed under federal regulations. For day trips, bus drivers typically face a maximum of 15 hours on duty.

The drive from Bristol to Boston takes about five hours.

When the charter bus slammed into the overpass about 7:30 p.m. Saturday, it sheared back the roof, broke windows, and left students and chaperones trapped for up to an hour.

Massachusetts officials said the overpass did not sustain any structural damage, but that it was due to be rehabilitated next year anyway.

Several people familiar with the Boston area said they weren't surprised the crash happened on Soldier Field Road. The low clearances apparently have been problematic for decades with out-of-town bus drivers and new college students in moving trucks.

Adding to the problem, GPS systems and online maps don't always include bridge heights or other logistical restrictions.

Sara Lavoie, a spokeswoman for the Massachusetts Department of Transportation, told the Boston Globe the Central Artery tunnel system detects about 10,000 overheight vehicles a year.

"These vehicles tripped our sensors that many times. The sensors trigger a message on our electronic signs and say either, 'Take next exit' or, 'Stop await police,' depending on the overheight's location," Lavoie wrote in an e-mail to the newspaper.

Desiree Goodwin, a librarian with the Harvard Graduate School of Design, had dinner with the Destined for a Dream group just before the crash, she told The Inquirer.

"They were very, very sweet, just wonderful kids and excited about this adventure of touring a college campus," she said Sunday.

Goodwin, 48, a Philadelphia native, met Destined for a Dream founder Erica Walls-Hill a few years ago, and the two kept in contact. When Goodwin heard about the crash on TV, she and her boyfriend rushed to the hospital to help.

They stayed until 5 a.m., enlisting student groups at Harvard to help arrange hotel rooms, transportation, and food for those who didn't stay at the hospital.

Goodwin worries the crash will turn a positive trip into a traumatic memory.

"It wrenched my stomach to see that," she said.

She said she hoped it didn't "make parents afraid of college visits."

In 2012, the charity took students to visit Arcadia University in Montgomery County and Georgetown University in Washington, according to its website. The organization also provides financial literacy, tutoring, healthy lifestyle seminars, and after-school activities.