A photo on a local conservative club’s Web site shows a single-engine plane flying over the area with a banner that reads, “Thank you Camdenton — Get Lost A.C.L.U.”

Last month, Nanette K. Laughrey, a United States district judge in Jefferson City, Mo., issued a preliminary injunction ordering Camdenton to discontinue “its Internet filter system as currently configured.” She wrote that “any new system selected must not discriminate against Web sites expressing a positive viewpoint toward LGBT individuals.”

The judge noted that a girl who had complained to the A.C.L.U. — identified only as Jane Doe in the lawsuit — was afraid to ask school officials to have sites unblocked because it would “draw attention to her and make her the subject of further taunting.”

The A.C.L.U. identified 41 Web sites supportive of gay people that were blocked by URLBlacklist, and then tested those 41 sites on five other Internet filters.

None of the 41 were blocked by the five other filters tested.

In traditional school censorship cases, civil liberties lawyers have relied on professional librarians as expert witnesses. For this case they found Mr. Hinkle, of Davenport, Iowa. He has no college degree, but in 1999, at the age of 20, he developed an Internet filter now used by more than 1,000 school districts.

The civil liberties lawyers spent weeks looking for someone who would be able to explain in plain English how an Internet filter could be configured to discriminate. “We found experts who understood the technology, but we didn’t know what they were talking about,” Mr. Rothert said. “David was able to explain so we could understand, and if we could understand him, a judge could.”

The way it worked: the URLBlacklist filter classified gay organizations in the “sexuality” category. The sexuality filter also screened out pornography. As a result, when URLBlacklist filtered pornography, it also filtered Web sites supportive of gay causes.