SIMI VALLEY, Calif. — A multi-vehicle crash involving a bus with Bakersfield students from Horizon Elementary on board prompted the closure of the westbound 118 Freeway in Simi Valley on Tuesday morning.

The crash happened shortly after 9 a.m. near Madera Road, the Ventura County Fire Department said, adding that fifth grade students were on a field trip to the Ronald Reagan Presidential Library.

The pileup involved 43 people in four vehicles - 33 minors and seven adults, investigators said. Four people, three of whom suffered minor injuries, were transported to a hospital, according to the Fire Department. A fifth patient declined to be taken to a medical center.

The Fire Department's Urban Search and Rescue team used wood and struts to shore up the unstable bus, while other firefighters used ladders to help the passengers get out safely.

According to Greenfield Union Elementary School District's Superintendent Ramon Hendrix, three students were taken to a local hospital with minor injuries and the rest of the students walked away with no injuries. The three students with minor injuries were evaluated and released Tuesday morning.

Hendrix also said that all students and staff were not able to finish the field trip and would be taken home via Classic Charters after conducting interviews with CHP officials.

According to school officials all parents were contacted and the field trip will be rescheduled.

The cause of the pileup is still under investigation.

DEVELOPING: More details will be added to this report as they become available.