Winston-Salem, N.C. — For as long as students have been taking test-preparation classes, the practice has been mysterious. Just how much can test prep help a student? What kind of students benefit the most? And is all test prep created equal?

Claudia Buchmann explored those questions here at Wake Forest University’s “Rethinking Admissions” conference on Wednesday. Ms. Buchmann, an associate professor of sociology at Ohio State University, presented findings from a recent study on the role that test preparation plays in admissions. She and her co-authors found that all kinds of test prep (including books, computer software, high-school courses, and private courses) tend to increase students’ SAT scores.

Yet private instruction, the priciest option, was the most effective kind: On average, students who took private SAT-prep classes scored 60 points higher on the exam than did those without such classes. “Yes, test prep pays off,” Ms. Buchmann said.

Several other factors may also explain why wealthier students are more likely than their lower-income peers to score high on the SAT and enroll in college, Ms. Buchmann acknowledged. Nonetheless, her conclusion about test prep was clear: “It functions as one tool for advantaged families to ensure that their children are staying ahead in the competition for college admission.” —Eric Hoover