HUNTSVILLE, Texas (AP) — Death row inmate Marvin Wilson spent his school years in special education classes, got failing or near failing grades and several times received "social promotions" to the next grade level until he finally quit school in the 10th grade.

Wilson's lawyers have asked the U.S. Supreme Court to overrule state and lower federal courts that questioned the validity of a psychological test that pegged his IQ at 61, below the threshold of 70 that would suggest he's mentally impaired and thus, ineligible for capital punishment.

If Texas proceeds with Wilson's scheduled Tuesday execution in Huntsville, he "will own the grisly distinction" of becoming the lowest-IQ Texas prisoner put to death despite the Supreme Court ruling banning the execution of mentally impaired inmates, his lead attorney, University of Maryland law professor Lee Kovarsky, wrote in an the appeal.

State lawyers contend that the 2004 IQ test was conducted by an inexperienced intern and was faulty, and that all other tests and assessments pegged the 54-year-old Wilson's IQ above 70.

"Wilson is a savvy, street-wise drug dealer and thief who exhibited skills surpassing those of a mentally retarded person from a young age," Edward Marshall, an assistant Texas attorney general, told the justices.

Wilson was convicted of murdering 21-year-old Jerry Williams in November 1992, several days after police seized 24 grams of cocaine from Wilson's apartment and arrested him. Witnesses testified that Wilson and another man, Andrew Lewis, beat Williams outside of a Beaumont convenience store and that Wilson, who was free on bond, accused Williams of snitching on him about the drugs.

Witnesses said the Wilson and Lewis then abducted Williams, and neighborhood residents said they heard a gunshot a short time later. Williams was found dead on the side of a road the next day, clad only in socks. He had been severely beaten and shot in the head and neck at close range.

At his trial, Lewis' wife testified that Wilson confessed to the killing in front of her, her husband and his own wife.

"Don't be mad at Andrew because Andrew did not do it," Lewis' wife said Wilson told them. "I did it."

Lewis received a life prison term for his involvement.

Wilson was arrested the day after Williams' body was found when he reported to his parole officer on a robbery conviction for which he served less than four years of a 20-year prison sentence. It was the second time he had been sent to prison for robbery, and his mental competency wasn't brought up in either case.

Wilson has declined to speak to reporters in recent weeks, although on a website where prisoners seek people to write them and provide money, he described himself as wearing "a nice smile whether I'm jolly or sad."

"Now I am not willing to try and convince anyone that I'm not guilty as charged because I realize that if I had not been selling crack then I wouldn't have been in the position to be set up as I have been."

His attorneys suggest that a gray hair from someone white that was found on Williams' body could mean someone else killed him. Williams and Lewis are black.

Ed Shettle, the Jefferson County assistant district attorney who prosecuted Wilson, dismissed the theory as a "red herring."

"Marvin had a motive to murder this man," Shettle said last week. "He was responsible for Marvin getting a couple of drug indictments."