A school bus veered off the road Tuesday and plunged 20 feet (six meters) into a ditch in southwestern Germany, injuring 12 children, four of them seriously, police was quoted as saying by Associated Press. Paul Wissler, a spokesman for regional police, said some 35-40 children were on the bus at the time of the accident, but all were able to free themselves before rescue services arrived. Four children were taken to the hospital for treatment of serious but not life-threatening injuries, he said. Wissler said the bus slipped off the snowy road on a hairpin turn near the village of Bonndorf in the Black Forest. It broke through a barrier, flipped on its side and slid down a hillside, coming to a stop in a stream bed, he said. It is not immediately clear what caused the accident, police said. Further details on the ages of the students and the condition of the bus driver were not immediately available.