Boston Magazine recently gathered data from all 230 public high schools in the Greater Boston area to determine which schools are the best, and they determined that Dover-Sherborn is the third best school in the state.
Data gathered included test scores, number of AP classes, graduation rate, percentage of students who continue on to college, as well as the number of sports teams and clubs. Per pupil spending and student to teacher ratio were included in the rankings, but less weight was given to them as recent studies have shown that those are not as important factors in quality of education.
You can access the main page of Boston Magazine's Best Schools and then go from there to see some other charts related to the rankings with various types of data. You can also go directly to the Top-50 list, to see who was ranked ahead and just below Dover-Sherborn.