A federal grand jury here is investigating an alleged scheme in which officials of a Los-Angeles-based office supply firm convinced officials of other companies, including M.S.Ginn Co. of Washington, to buy office good at inflated prices in exchange for kickbacks.

In one such instance, a Los Angeles woman, Terry Scott, 28, pleaded guilty in federal court here yesterday to attempting to obtain money under false pretenses. She admitted pocketing more than $5,000 from inflated contracts worth $100,000 that were won by Qualtra Corp. to sell ballpoint pens to Ginn's.

Scott, brushing back tears, told U.S. Magistrate Henry H.Kennedy Jr. that when she started work at Qualtra three years ago, "I thought I'd be successful in the company."

She said "senior officials" in the company told her that the way to success was to locate firms that wanted to buy aupply to success was to locate firms that wanted to buy suppln value so that there would be extra money left over after Qualtra covered its costs.

According to the criminal information to which she pleaded, Scott, from August 1976 to August 1978, convinced Ginn's purchasing agent, Alfred Coleman, to agree to buy ballpoint pens at prices which were "substantially higher" than the Washington firm normally paid.

In exchange for approving the inflated contracts, Scott gave Coleman color television sets, cameras and money "which he concealed from Ginn's," according to the information. Coleman has since left Ginn's.

Efforts to reach Coleman or Ginn's officials for comment yesterday were unsuccessful.

Assistant U.S. Attorney Peter H. Goldberg told Kennedy that Scott is cooperating with the government's investigation. He said later that more criminal information and indictments are expected in the investigation.

"What I've done was a mistake," Scott told Kennedy, adding that she hopes to return to California to pursue a master's degree in psychology at the University of Southern California.

Kennedy placed the woman on two years' probation and ordered her to make restitution of $5,000 un the case in monthly installments.