Sioux City police say students who pelted a school bus with snowballs caused a collision that injured 21 other students.
Sgt. Brad Bollinger says the accident occurred about 3:05 p.m. Tuesday in front of West High School. Bollinger says the driver stopped the school bus when the snowballs hit it. It was struck in the rear by a city transit bus that couldn't stop in time.
The buses were carrying students home from West High and West Middle schools.
The students suffered mostly bumps and bruises. One student was taken by ambulance to a hospital. The bus drivers weren't hurt.
School spokeswoman Alison Benson says throwing snowballs at a bus would violate the district's code of conduct and that students could be punished.