A D.C. Superior Court jury yesterday lowered from $640,000 to $173,000 the amount of damages originally awarded to a girl raped three times by a janitor in a city school in 1974. The jury verdict in the new trial was directed against the janitor and not the city, which would have been responsible for paying the original award.

A new trial had been ordered in the case after the trial judge determined that the $640,000 verdict was based on "emotion rather than a calm, dispassionate weighing of the evidence."

The second jury, composed of four women and two men, made the award after deliberating late Wednesday afternoon and until 3 p.m. yesterday, according to an attorney who represented the victim, now 13 years old.

The janitor, Ronald G. Harrison, 24, was sentenced in 1975 to three to nine years in prison for carnal knowledge and is now in a penitentiary in Virginia, according to the attorney, Samuel M. Shapiro.

"We have not even looked into what assets Mr. Harrison may have" to pay the award, Shapiro said in a telephone interview.

"Obviously he's in prison so he may not have very much money," said John Suda, an assistant corporation counsel who tried the case for the city.

The girl was raped in 1974 between April 25 and May 11 while she was in fourth grade at the Bunker Hill Elementary School at 14th Street and Michigan Avenue NE. All the assaults occurred in a second-floor bathroom.

According to court records, the girl was frightened by threats made by Harrison and did not tell her parents or school officials about the incidents immediately after they occurred. In June, 1974, after a doctor told her she had gonorrhea, the child told her mother about the assaults.

Two of the incidents occurred under similar circumstances while the girl was waiting to use a bathroom. The other attack occurred while the girl was delivering a note for her teacher and Harrison forced her into the bathroom.

The victim's parents suied the city, the school principal, a guidance counselor, the girl's teacher and the janitor, Harrison. The suit claimed the school system was responsible for protecting students against such acts by its employes.

In June, 1976, the jury awarded the girl $640,000 in a verdict against the city, its employees and Harrison. The city would have been responsible for the amount. Judge James A. Belson, who presided at the trial, ruled last October that the award was excessive and ordered a new trial.Judge George H. Goodrich presided at the retrial.

Shapiro said he presented an almost identical case. Suda said the city emphasized in the retrial that the city was not negligent because school officials had no reason to believe such an incident would occur. No assaults had been reported at the school prior to the incidents involving the girl, Suda said.

Shapiro said no decision has yet been made about an appeal of yesterday's award.