A Fairfax County secondary school teacher has been arrested by FBI agents and charged with "loan sharking" and extortion for allegedly attempting to collect more than $25,000 for a $1,500 loan.

Robert Singletary, 45, an eighth-grade math teacher at Robinson Secondary School, was charged Monday night in connection with a loan he allegedly made to Laurence Griffin of Washington. He has been placed on administrative leave with pay pending an investigation by school authorities, a school official said yesterday.

Singletary, who listed his residence as 2300 Good Hope Rd. in Southeast Washington, appeared before a U.S. District Court magistrate here yesterday and was released on his own recognizance. A hearing was set for March 21.

According to an affidavit filed as a complaint against Singletary, Griffin, a social aquaintance, borrowed about $1,500 between January 1981 and last October at 25 to 30 percent interest due every two weeks.

Last Oct. 15, having quit his job in Silver Spring to draw from his retirement and sick-leave pay, Griffin allegedly went to Singletary's home to make a cash payment of $2,700. According to the affidavit, Singletary told Griffin that he still owed $3,400, which he could either pay in another lump sum, or by alternate fortnightly payments of $280 and $150 through April 1985--a total of $14,040 in addition to more than $11,000 Griffin had already paid.

At that meeting, according to the affidavit, Singletary allegedly told Griffin that he had "other people to whom he has to answer," people who had photographs of Griffin and would know how to find him if he didn't pay.

Robinson Principal Robert C. Russell called Singletary a "good teacher. . . He's been with us 12 years, the kids like him very much, and his fellow teachers like him."