In what is perceived to be a setback for the prosecution, a key witness in the murder trial of Michael Skakel admitted in a Stamford, Conn., courtroom Wednesday that he was high on heroin when he told a grand jury that the Kennedy cousin confessed to killing his neighbor, Martha Moxley, in 1975. The surprise testimony by witness Gregory Coleman came on the first day of a hearing (which is expected to last through the week) in which Judge John F. Kavanewsky will decide whether or not there is enough evidence for the case against Skakel to proceed to jury trial. Coleman had been a classmate of Skakel’s at a school for troubled youths in the late ’70s. Speaking before a 1999 grand jury, Coleman quoted Skakel of having boasted at school, “I’m going to get away with murder. I’m a Kennedy.” (Skakel is the nephew of Robert Kennedy’s widow, Ethel.) In court on Wednesday, Skakel’s lawyer, Michael Sherman, asked Coleman, “Are you on drugs now?” “No. Give me a urine test,” replied Coleman. “No, thanks,” said Sherman.