A few weeks ago, boarding school students in the Northeast began moving into their dorm rooms armed with all the usual clatter. The Indian print tapestries, the athletic gear, the odd beanbag chair. But a few of them arrived with some extra equipment: their parents. In towns like Lakeville, Washington Depot, and Sharon, Conn.; Millbrook, N.Y.; Deerfield and North Andover, Mass.; and Newport and Middletown, R.I., some families are buying or renting houses and apartments to be close to their children, who are living in dorms.