A 16-year-old junior at Oakton High School was killed yesterday and two other teenagers were critically injured in a two-car collision in the Reston area of Fairfax County, county police said.

Police said Jennifer Parnell, of the 2900 block of Brook Mill Court in the Herndon area, was killed about 11:15 a.m. in the crash at Lawyers Road and Myrtle Lane.

Police said in a written statement that she was trying to turn left from Myrtle Lane onto northbound Lawyers Road when her sedan "for an undetermined reason" pulled into the path of a southbound station wagon.

The two critically injured teenagers were riding in her car, police said. They identified one as Patrick Hagerty, 17, of the 12400 block of Wendell Holmes Road in the Herndon area. The name of the other was withheld until his relatives could be notified, police said.

Curtis Vaughn, president of the PTA at Oakton, said Parnell had been a cheerleader at the school and before that had been involved in gymnastics.

Sandy Reynolds, director of student activities at Oakton, said personnel at the school were "all devastated right now." And, she added, "I know that all the kids will be on Monday," when school resumes.