Responding to a nationwide measles outbreak, the American Academy of Pediatrics has decided that all middle-school-age children should receive a second measles shot. The decision was a change in position for the prestigious medical group, said Dr. Milton Schwarz, president of the Orange County chapter. Twenty-four states, including California, have reported 5,569 cases this year, a 300% increase over the same period last year, the National Centers for Disease Control in Atlanta said. Among the hardest hit have been children in the Los Angeles area, where eight have died from complications. The academy recommends that all children receive a booster "MMR" shot for measles, mumps and rubella as they enter junior high. The academy also recommended that anyone born after 1957 be re-vaccinated.