HOUSTON (AP) _ A flatbed truck hauling cars crashed into a school bus, ripping a gash in its backside and seriously injuring two teen-age girls who were sitting in the back of the bus.

About 20 other children suffered cuts and bruises in the crash, which happened on rural U.S. Highway 290 near Chappell Hill, 60 miles northwest of Houston.

The bus driver had pulled over in the eastbound lane to unload children Tuesday afternoon and the bus' red warning lights were flashing when the truck slammed into its rear left corner, said Mike Cox, a spokesman for the Texas Department of Public Safety.

``It was pretty torn up,'' said Tom Flanagan, an emergency rescue worker on the scene. ``The bus was in a culvert, nose in.''

The most seriously injured girls, ages 17 and 16, were airlifted to a Houston hospital. The older girl was in critical condition early today with head and abdominal injuries, a spokeswoman said. The younger girl was in stable condition with a severe cut on the back of her head.

Twenty-eight children, ages 6 to 18, from the Brenham Independent School District were on the bus when it was hit, said DPS spokeswoman Tela Mange. About 45 children are assigned to the bus route, but many already had been dropped off.

The bus driver, who wasn't identified, was treated for minor injuries.

The truck driver, Wallace James Stage Jr., 37, was treated for cuts.

Authorities would not speculate about possible charges until an accident investigation was conducted.