Two teen-agers returning home from a trip to Ocean City were killed early yesterday when their sports car slammed into the back of a tractor-trailer that was parked on the westbound shoulder of U.S. 50 near the Bay Bridge.

The teen-agers' car apparently veered from the highway just before the exit for Sandy Point Park about 5:30 a.m. and became lodged beneath the truck's trailer, authorities said. Anne Arundel County firefighters used an airbag system to lift the tractor-trailer off the ground in an unsuccessful effort to save the teen-agers.

"The car was almost completely under the trailer," said Anne Arundel County Fire Division Chief John Scholz.

Tiffany N. Wells of Clinton and Dustin S. Pion of Waldorf, both 16, were pronounced dead at the scene. They were returning to their Southern Maryland homes from the Eastern Shore, where they had celebrated the end of school with friends, according to Wells' grandmother.

Wells was driving the red Nissan 300ZX at the time of the accident, but the car was not registered to Wells or Pion, said Cpl. Greg Prioleau of the Maryland Transportation Authority Police. He declined to be more specific, saying that investigators were determining the relationship of the car's owner to the teen-agers.

Mary Ellen Wells, Tiffany Wells' grandmother, said yesterday that her granddaughter had been driving for about a year and that she was a responsible driver. Wells and Pion attended Thomas Stone High School in Waldorf, she said.

The driver of the tractor-trailer, Jose Luis Wagner, 42, of Chesapeake, Va., was not injured, Prioleau said. He said police are trying to determine why Wagner was stopped in an area that was posted "no stopping anytime."