When college admission committees evaluate their candidates, they don’t just focus on the “numbers” (GPA, rank, test scores) but also on the factors that contribute to these numbers. So, when a high school is especially rigorous and/or competitive, admission officials realize that a student in the second decile may, in fact, be stronger than a student in the first decile elsewhere. The admission officials also note if the student has a “rising record” (i.e., grades improved after freshman or sophomore year) or if there were extenuating circumstances that might have affected the grades and thus the class rank (e.g., the student took an exceptionally demanding course load or missed school due to a serious illness or family crisis).
Thus, admission committees don’t look at a rank in isolation, and they realize that at some cut-throat high schools, a single B+ can send a student from the top tenth down to the second one. But, nonetheless, colleges do like to boast that they have high percentages of students who were in the top tenth of their graduating classes. So it’s definitely an advantage to land there, yet it’s not an automatic deal-breaker for those who don’t.