From the article:
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Although standardized test scores were initially implemented to put students on a level playing field for aptitude assessment in the college admissions game, they have since become a divisive factor among applicants. Students with more money to spend on tutors and prep books have an advantage over others who cannot afford to pour money and resources into their preparation. This socioeconomic divide polarizes students, equally prepared for college, by their SAT/ACT scores and puts underprivileged applicants at a disadvantage when up against their privileged peers in the applicant pool. Consequently, the tests have been criticized for failing to serve as an indicator for success in college; a recent National Association for College Admissions Counseling (NACAC) study has confirmed that view.
In reality, differential access to test prep explains very little of the SES gap in standardized test scores, as discussed in the following blog post:

The SAT, Test Prep, Income and Race
by Alex Tabarrok
March 11, 2014