Popular rappers like the Fresh Prince are urging kids to stay in school, and now a rap group called the Lifers has a sterner message: Stay out of prison. The Lifers, the musical subset of a long-standing support group for ghetto kids, are 15 inmates nine rapper emcees, a trumpeter, a bass player, and four vocalists in Rahway's East Jersey State Prison, all of them serving extended or life sentences. Proceeds from The Real Deal, the Lifers' EP on Hollywood Records, will go to the Juvenile Awareness Program, which members organized in 1975. The program is known for its ''Scared Straight'' campaign (which in the mid-'70s sent crime-prone kids to prison for a day) and more than a dozen documentaries, including some Academy Award winners. In stark contrast to rappers like Ice Cube, who glorify prison life as ''a party,'' the Lifers depict it as a hell of menial labor, violence, and AIDS. Asked to comment on Cube's ''gangsta'' rapper attitude, Lifer Quhhar Saahir says, ''There's nothing great about prison. He [Cube] ain't been in here.''