The sex 'n' swank allure of NBC's series "L.A. Law" is apparently having a strong pull on high school students--at least in San Mateo County, Calif., where teen-agers are choosing overwhelmingly to become lawyers. The San Mateo County school system's annual career inventory survey showed that law, which had never jumped higher than third among desired occupations, was far and away the No. 1 career choice. "The legal life style (as shown on "L.A. Law") is so glamorous," said survey supervisor Sheryl Pomerenk. "They all drive BMWs and have affairs." But Pomerenk dismissed the students' choice as a mere trend. "The same thing happened the year 'Raiders of the Lost Ark' came out," Pomerenk told the San Mateo Times in an article published Saturday. "Anthropologists moved way up."