A Prince George's County Circuit Court jury has awarded a total of $843,000 to two Upper Marlboro families in a suit over an accident that killed one teen-ager and injured another in 1978. Court sources called the award unusually large for such a case.

The jury's verdict, returned last Friday, held Riverdale Baptist School in Upper Marlboro responsible for the death of Lili-Ann (Peaches) Elm, 16, who was killed when the dirt bike she was riding crashed into the rear of a disabled truck owned by the school on Aug. 8, 1978. Gail M. Kidwell, 14, a passenger on the bike, was injured.

The jury awarded $528,000 to the Elm family and $315,000 to Gail Kidwell and her mother for personal suffering and medical expenses, according to Daniel Shea, attorney for the plaintiffs.

Robert Thompson, business manager for the school, said the truck had been stolen, and when school employes found it, it had a hole in the gas tank, making it impossible to drive. The plaintiffs argued that the school was negligent for leaving the vehicle by the side of Old Marlboro Pike without flares or other warning signals.

Thompson said school officials were shocked at the verdict and called the size of the award "absurd" since the jury found no malice on the part of the school. Thompson said he felt the girls were negligent for riding the bike without a light on the dark road.