20 people injured when school bus transporting students to a day trip is rear-ended by an out-of-control semi-truck

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Authorities say 20 people were hurt, four of them seriously, when a semi-truck crashed into the back of a school bus in south central Wisconsin.

The bus was transporting students and chaperons from a school in the Milwaukee area to Wisconsin Dells around 10:30am when it was rear-ended by a truck. 

Minutes before the crash, Wisconsin State Police had received calls about a semi-truck driver that was driving erratically, the Journal Sentinel reports.  

20 people were hurt Wednesday morning when a semi crashed into the back of a school bus in south central Wisconsin  

The Wisconsin State Police had received calls about a truck driver driving erratically down Interstate 39 minutes before the crash  

Police said about 30 people were on the bus, including five chaperons and the bus driver, when the crash happened. The students were on their way to a day trip in Wisconsin Dells 

According to authorities, the school bus had pulled onto the shoulder of Interstate 39 near Lodi, about 95 miles northwest of Milwaukee, because of a mechanical issue when it was hit by the truck. 

Photos from the crash show the left side of the back of the school bus completely destroyed. The semi suffered extensive damage on its front side. 

Police said about 30 people, including five chaperons and the bus driver, were on the bus, and 20 of them were hurt. Not everyone injured was a student, the Journal Sentinel reports.

 Two of those injured had to be airlifted to the hospital.  

A spokesperson for UW Health said nine patients were being treated at its hospitals. Two were taken to The American Center site and seven to UW Hospital. Their conditions are being assessed. 

St.Mary's Hospital in Madison confirmed that 11 patients were being treated at its facility.  

Authorities said the bus had pulled over on the shoulder of the road because of mechanical issues when the semi-truck ran into the back of the bus 

20 people who were on the bus were taken to area hospital. Authorities said the driver of the truck did not appear to be hurt 

The students are from Hope Christian Schools in the Milwaukee area. It's not clear what ages the children were or which school in the district they attend 

Another bus from Milwaukee was sent to pick up the remaining passengers. The driver of the semi-truck did not appear to be hurt, according to NBC 15. 

The students aboard the bus attend Hope Christian Schools in Milwaukee. According to the district's website, Hope Christian Schools is a network of eight college-preparatory schools Milwaukee and Racine. 

The school district has not commented on the crash. 

A portion of Interstate 39 was closed Wednesday morning after the accident, but reopened around 1:30pm.