POSTERS: The latest news in AIDS education is an unusual high school poster contest sponsored by the Red Cross and Cal State Fullerton. . . . Promos take a jovial tone, featuring a smiling cartoon of “Andi-Body,” who combats the AIDS virus. . . . But the purpose couldn’t be more serious. Contestants must research “How Do We Stop AIDS?"--which by last month had killed 719 people in the county. Because the topic is a sexually transmitted disease, students need parental permission to enter. . . . The contest deadline is today.