According to the Harvard Common Data Set, 95% of accepted students are in the top 10% of their high school class (http://oir.harvard.edu/files/huoir/files/harvard_cds_2013-14.pdf). I imagine that students not ranked in the top 10% of their class have some sort of other factor that mitigates their ranking – recruited athlete, legacy, etc.
While Harvard doesn’t consider a student’s rank when making their decision, a student’s recommendation writer’s choice of words will usually allow an Admissions Director to understand where the student stands in the “pecking order” at their high school. After all, a teacher is not going to say the same things about a student who is ranked in the second decile, as they would for someone ranked second in their class. In the end, a student ranked in the second decile is probably rejected not because of their ranking, but because their teachers did not write a gob-smacking recommendation for the student.