All incoming kindergarten students and older pupils transferring to Los Angeles County schools will be required to undergo tuberculosis skin testing beginning this fall, health officials announced. About 200,000 students will be affected by the new policy, meant to help eradicate Los Angeles' persistently high tuberculosis rate. Los Angeles-area residents suffer from tuberculosis at nearly twice the national rate, partly because of the large immigrant communities in the county, said Dr. Paul T. Davidson, the county's director of tuberculosis control. Mandatory testing for tuberculosis--a contagious bacterial infection that usually attacks the lungs--follows earlier requirements that children be thoroughly immunized against other common diseases before being allowed to enroll. A state law enacted four years ago allows county health directors to add tuberculosis tests to the list.