President Clinton is planning to choose as his next surgeon general Dr. Henry Foster, the former head of a predominantly-black medical school who distinguished himself by working with teenagers, sources tell the Associated Press. The announcement is expected this week. If Foster is chosen, he is expected to lead a national effort to curb teen-age pregnancy. Foster is the former acting head of Meharry Medical College, in Nashville, Tenn. He directed the school's "I Have A Future Program," aimed at curbing teenage sex, by giving teenagers job training, boosting their self-esteem, and instructing them in sexual responsibility. Foster will replace Joycelyn Elders, who was fired in December after she advocated teaching children about masturbation.