What does is it take to be the best high school in Tennessee?Just ask Dr. John Ash, founding principal of Central Magnet School in Murfreesboro, judged by US News & World Report to be the Volunteer State’s highest-performing secondary institution.Central Magnet serves some 1,260 students in grades 6 through 12, part of the 40,000-student Rutherford County system which has seen explosive growth over the past several years as a bedroom community to Nashville and a dynamic business and education hub in its own right.Besides being scored best in Tennessee, Central earned 37th place among the thousands of American high schools and won first-place among U.S. magnet schools.Speaking at The Rotary Club of McMinnville last week, Ash emphasized school “culture” and “habits of excellence” as basic ingredients in producing top academic results. On the culture side, he noted only a half dozen or so of the student lockers are actually secured by padlocks.“In seven years there’s been no theft,” he said. At the bulk charging stations for cellphones, students can leave their wireless devices without fear they’ll go missing.Among the many measures of achievement at Central Magnet, the school’s 193 graduates this year earned more than $10 million in accepted scholarships, not counting the usual student financial aid and scholarship offers declined in favor of better deals.