A school bus driver and 12 students from the Fairfax Christian School were injured, three seriously, yesterday when the bus in which they were riding ran off a two-lane road plunged down an embankment and hit a tree in western Fairfax County, police reported.

The injured passengers, one of them the 5-year-old daughter of Redskins quarterback Joe Theismann, were treated at Commonwealth Doctors and Fairfax hospitals for head, chest and leg injuries, cuts and bruises, accordingg to hospital spokesmen. Nine of those injured, including Theismann's daughter Amy and the school bus driver, were admitted to the two hospitals, the spokesmen said.

According to a police spokesman, the students were being driven home at 3:20 p.m. when the accident occurred. The bus traveling along Shirley Gate Road approaching Lee Highway, ran off the roadway, plunged down the embankment and hit a 60-foot pine tree broadside, the spokesman said.

Police said they had not determined the cause of the accident, which was still under investigation.

Robert Thoburn, owner and operator of the private school at 11211 Popes Head Rd., said the area in which the accident occured has "a very bad ditch on a winding road."

Thoburn said the yellow Dodge maxi-wagons, which serve as school buses, can carry up to 17 students.

Fourteen students were aboard the bus involved in the accident, police said. Two, including Theismann's 8-year-old son Joe Jr., were uninjured, police and hospital officials said.

Thoburn said the 34 school buses are equipped with seat belts, which the students are required to use.

He said the bus driver, Kathleen Brewer, 26, of 12065 Gray Wing Sq. are og Fairfax, has been driving the vehicle for a year. Brewer is an elementary teacher at the school, he said.

Yesterday's school bus accident was the first in the 17-year history of the private school, Thoburn said. The school, which serves students from kindergarten through the 12th grade, has an enrollment of 517, he said.

Persons admitted to Commonwealth Doctors Hospital and their injuries were Amy Theismann, facial lacerations; Renee Raspen, 8, multiple cuts; Rosie Bradley, 10, chest injuries; and Jerrod Dalton, 10, head lacerations, according to a hospital spokesman.

Admitted to Fairfax Hospital were Brewer, multiple fractures; Joanna and Jennifer Walker, 5-year-old twins, head injuries; Kevin Sullivan, 9, fractured leg; and Greg A, Croson, 11, fractured left shoulder and concussion, a hospital spokesman said.

Four students were treated and later released, the spokesmen said.