As the weeping friends of Lisa Bates lined a Fairfax courtroom yesterday, Roger O. Simmers, 22, of Reston pleaded guilty to hit and run and drunk driving charges arising from the accident in which the 17-year-old Reston girl was killed in December.

Circuit Court Judge J. Howe Brown then ordered Simmers, who had been free on $2,000 bond, to await his Sept. 27 sentencing in jail, an action that caused tears from the defendant and stunned looks from his parents and girlfriend standing nearby.

Barry Schneiderman, Simmers' attorney, had asked that Simmers, a car mechanic who had one previous reckless driving conviction, be allowed to remain free on bond.

On the day the fatal accident occurred, prosecutor Lorraine Nordlund said, Simmers had been stopped earlier in the evening by a U.S. Park Police officer on the George Washington Memorial Parkway, but no citation was issued. Investigators have been unable to determine which officer stopped him, why he was stopped, or why he was not cited.

In the hallway outside the courtroom yesterday, the dead girl's parents and sister were mobbed by a dozen consoling and jubilant teen-age friends of Bates.

Simmers pleaded guilty to felony hit-and-run and misdemeanor driving while intoxicated. The guilty pleas were in exchange for reduction of a felony manslaughter charge to the DWI charge. He faces a maximum of six years in prison.

Lisa Bates' parents said they had been consulted by Nordlund about the plea bargain, and approved it.

"We are pleased with the results," said Lisa's father, Glenn Bates.

Asked why so many of Lisa's friends had come on a cool, sunny, summer morning to spend three hours in a courthouse, Patricia Bates responded: "Did you know our daughter? That's why."

Six school busloads of students from South Lakes High School, where the girl was a senior and a cheerleader, attended her funeral last December.

The girl was killed in the early morning hours of Dec. 8 when the car in which she was a passenger was struck broadside by a car driven by Simmers at Rosedown and Kings Lakes Drives in Reston. Simmers allegedly ran a stop sign before hitting Bates' car.

Immediately after the accident, Simmers left the scene and walked to his girlfriend's house, where he was found by police, Nordlund said in court yesterday.

Nordlund recounted yesterday how Simmers and a friend had driven from a party to a bar to the homes of several friends the night of the accident, drinking at each stop, before the accident.