Your school doesn't rank for a reason. They do not believe that the rankings give an accurate picture of applicants academic strengths. Dartmouth will want to know that you are a high ranking student and depending on how your school profile is written, that could present a problem. However, your other stats are amazing. I honestly think your biggest problems is not rank but ECs. Grades/Stats only get students so far. Write a great essay and I think you have a pretty decent chance.