Casting about to try to determine what makes a good school good, the Wilson Quarterly this fall drew up a list of the high schools with the most semi-finalists in the 1978 National Merit Scholarship test, a nationwide test given to high school juniors. The result was a list of great diversity in size, ethnic composition, location and median college board scores. "But," the magazine observed, "the similarities were even more striking. "All assign 'heavy' amounts of homework, and all have little faculty turnover and low student-teacher ratios. All kept their basic curricula more or less intact during the 1960s. Half the schools choose their students by means of tough exams and/or admissions procedures. The other half are selective by accident: They are located in prosperous suburbs where many parents are college graduates and most care deeply about their children's education," according to the magazine, which is put out by the Woodrow Wilson International Center for Scholars here. In addition, the students and teachers share a strongly felt school spirit. "Does Exeter's 'timeless vision' (as its catalog puts it) account for its No. 1 ranking?" the magazine wondered. "An admissions dean at rival Andover thought not. Students taking the National Merit Test, he sniffed, compete only against other students in their state, and New Hampshire has a notoriously poor public school system."