(FORTUNE Magazine) – Boarding school enrollment is down, and the trend is reshaping even the most elite institutions. In 1992 prep schools played home to 5.5% fewer boarders than in 1987. At the same time, 8.1% more day students attended private schools (see chart above). Demographics explain much of the shift. The high school population, which includes most boarders, dropped by almost one million between 1986 and 1991. Heather Hoerle, director of marketing for the National Association of Independent Schools, points to another reason: ''The legacy families that were once largely responsible for filling up these schools are reproducing at a low rate and aren't even replenishing the stock.'' Some private schools, like St. George's in Newport, Rhode Island (1993-94 tuition: $19,700), have kept enrollment high with aggressive marketing campaigns that reach out to other groups, especially international students. Admissions director Jay Doolittle notes that 28 of the school's 324 students this year are foreign, vs. five of 322 in 1985. Hoerle, for one, believes foreign parents will keep sending their kids to U.S. boarding schools. Says she: ''The international community sees our private schools as entree to a world of power, excellence, and affluence.'' -- R.T.

CHART: NOT AVAILABLE CREDIT: FORTUNE CHARTS/SOURCE NAIS CAPTION: Fewer private school students board. . . But more go just for the day