Image copyright AP Image caption A former assistant principal is led away in handcuffs after a jury found her guilty

Eleven former school teachers have been convicted for their involvement in a scheme to falsify student test scores.

They changed wrong answers to demonstrate student progress, and some received performance-related bonuses.

The scheme was revealed after a local paper reported that some of the scores were statistically unlikely.

Eleven were guilty of racketeering and face lengthy prison terms, with a 12th acquitted, in one of the biggest ever cheating scandals in the US.

Thirty-five people were originally charged in 2013, with many pleading guilty and some testifying at the trial. Scores more were implicated.

"The cheating had been going on so long, we considered it part of our jobs," Jackie Parks, a former teacher who was a witness for the prosecution, told the New York Times in 2013.

The school system's superintendent, Beverly Hall, died last month of breast cancer, and never appeared at the trial after arguing she was too sick.

Image copyright AP Image caption Hall maintained her innocence

She insisted that she was innocent, but many accused her of pressuring the teachers to show improvements in scores which would unlock greater federal funding.

In 2009, she was named Superintendent of the Year by the American Association of School Administrators for her district's dramatic improvements in test scores and graduation rates.