A crowded school bus skidded off a bridge on a highway in southern Nepal, killing at least 20 students and two teachers, police said Friday. At least 57 others were injured.

The bus was returning from a school picnic late Thursday when it plunged off the highway near Mukundapur village, about 125 miles southwest of the capital, Katmandu.

Police official Len Pratap Sen said all of the students were teenagers. The 57 injured were treated at several hospitals, Sen said.

An initial investigation indicated the bus was speeding, he said.

Accidents are common in Nepal, where both roads and vehicles are poorly maintained and there are no speed limits on highways.