The bus veered off the road, overcorrected and rolled Monday morning on Highway 281 just south of Quincy, an agricultural region about 120 miles east of Seattle, said Lt. Scott Martin of the Washington State Patrol.
A medical airlift was called for the serious injuries, he said. Other injuries included bloody noses, scrapes and bruises.
All of the students were injured to some extent and most have been taken to Quincy Valley Hospital, said Grant County sheriff's spokesman Kyle Foreman.
There were 38 students and the driver on board, he said.
The driver apparently was not injured, and no other vehicle was involved.
The Quincy School District covers about 450 square miles of farmland in central Washington's Columbia River basin, home to thousands of acres of tree fruit and potato and onion fields. The district serves 2,500 students.
All telephone calls were referred to the district's superintendent, Burton Dickerson, who did not immediately return a telephone call for information about the crash.
Lt. Kandi Patrick of the Washington State Patrol told television station KOMO that roadways were wet at the time, but it's not known of that was a factor in the crash.
