The Supreme Court yesterday let stand the conviction and 25-year sentence of Terrence G. Johnson for the 1978 gunning down of two Prince George's County policemen. The justices, without comment, refused to hear arguments that Johnson was denied a fair trial and received ineffective legal help on appeal.

Johnson had been charged with murder in the shooting deaths of Officers James B. Swart and Albert M. Claggett IV in the Hyattsville police station in June 1978. But a jury found him guilty of manslaughter and illegal use of a handgun in Claggett's death, and not guilty in Swart's.

Johnson had been arrested in a theft case and taken to the station, where he grabbed Claggett's gun and began shooting, according to police. His 1979 trial exacerbated racial tensions in Prince George's; Johnson is black and the two officers were white.

At his trial, Johnson pleaded not guilty by reason of temporary insanity caused by repeated and severe physical and mental abuse by the police officers.

The appeal acted on by the high court contended that Johnson was denied a fair trial because his lawyers were not present during a meeting between the judge and a woman juror who reportedly complained about racial prejudice on the part of the jury foreman.