A former Fairfax County legislator selected to help monitor a court-ordered community service program for the teen-ager convicted of last year's armed takeover of Lake Braddock High School stepped down after a judge discovered that the man is executive director of the Gun Owners of America.

Judge Richard J. Jamborsky criticized prosecution and defense lawyers for failing to identify Lawrence D. Pratt as the head of the anti-gun-control lobby last May when the judge approved the community service program for James Q. Stevens, 18. Both sides said they were unaware of Pratt's involvement with the group.

"I do not believe that supervision of Mr. Stevens by the executive director of the Gun Owners of America gives the victims the reassurance to which they are entitled," Jamborsky said in a letter to the lawyers. ". . . I do not want him serving as the court's representative in monitoring Mr. Stevens."

Pratt's participation in supervising Stevens' community service also had been approved by the National Center on Institutions and Alternatives in Alexandria, which helped develop the program for Stevens. David W. O'Connor, who worked on Stevens' case for the center, said the center did not believe Pratt's work with the gun owners would affect Stevens' supervision.

"We felt that Pratt's role with Gun Owners of America is his political stand," O'Connor said.

Stevens' mother had worked for the Gun Owners of America, according to defense lawyer Guy O. Farley Jr., and had recommended Pratt for the committee, which also includes a policeman, a Sunday school teacher and two neighbors in the Burke community where Stevens lived with his mother.

Last November, Stevens, armed with a high-powered rifle that his mother had given him for Christmas and distraught over a girlfriend who had rejected him, stormed the high school and held 10 people hostage. No one was physically injured in the 21-hour siege.

Stevens, sentenced by Jamborsky to serve 4 1/2 years in prison on three counts of abduction and one firearms charge, has been in jail since the November incident and could be eligible for parole in about three months.

Jamborsky ordered Stevens to complete three years of community service work with mentally retarded children after his release from jail.

Pratt was one of five individuals selected to serve on a panel to supervise Stevens' participation in that community service program. Pratt did not return a reporter's telephone calls yesterday.

Pratt "indicated he would step aside" when he was notified of the judge's demand, Farley said. He added that a replacement for Pratt on the committee has not been named.

"The court felt that it could be perceived as being inappropriate," said Farley about Pratt's participation. "It's a plan that has to work completely under the court's direction," Farley said.

Jamborsky told both Farley and and Deputy Commonwealth's Attorney V. Britt Richardson Jr. in his letter that Pratt "should have been identified as executive director of the Gun Owners of America" at the sentencing hearing.

Jamborsky said he learned of Pratt's involvement from a recent newspaper article on an unrelated subject quoting Pratt as executive director of Gun Owners of America, a national organization headquartered in Falls Church and claiming a membership of about 150,000.