(DeForest) The Wisconsin State Patrol say at least 20 people were injured when a semi struck a school bus with students aboard on Interstate 39 Wednesday morning in Columbia County. State patrol officials say four people were injured when the semi hit the school bus at shortly after 10am at mile marker 122 near DeForest. Troopers say the injured were airlifted to a nearby hospital for treatment. Officials say the students are from a Milwaukee-area private school and that the bus was parked on the shoulder when it was struck. Officials say students were transported to the Wisconsin State Patrol DeForest Post to be reunited with their families.