BOSTON—Harvard University’s longtime admissions dean defended the school’s recruitment of prospective students in the first day of a landmark trial accusing Harvard of discriminating against Asian-American applicants.

On Monday, lawyers for the plaintiffs focused on internal documents showing Harvard sends targeted letters to high-school students who score well on the PSAT, encouraging them to consider applying to the prestigious school. The score thresholds vary by race.

In a recent admissions year, white students in 20 underrepresented states—which Harvard calls “sparse country”—received a recruitment letter if they scored 1310 or higher out of a possible 1600 on the combined verbal and math components, according to the plaintiffs’ exhibit. In all U.S. states, Asian-American women had to score at least 1350 to receive a letter, while Asian-American men had to score at least 1380.

The PSAT is considered a preview of how a student may score on the SAT.

Black, Hispanic and Native American high-schoolers nationally who scored at least 1100 received a letter, the plaintiffs’ exhibit showed.

Crafting a Class The racial makeup of Harvard's admitted class hasn't changed dramatically in recent years. The school says this reflects the stable percentages of applicants by race, while the plaintiffs say it's evidence of illegal race quotas. Admitted students Applicants % 70 % 70 60 60 All other, including white* 50 50 40 40 White 30 30 Asian- American Asian- American 20 20 Hispanic Hispanic 10 10 African- American African- American 0 0 ’14 ’16 ’16 ’12 2010 2010 ’12 ’14 Admitted students Applicants 70 % 70 % 60 60 All other, including white* 50 50 40 40 White 30 30 Asian- American Asian- American 20 20 Hispanic Hispanic 10 10 African- American African- American 0 0 2010 ’12 ’14 ’14 ’12 ’16 2010 ’16 Applicants Admitted students 70 70 % % 60 60 All other, including white* 50 50 40 40 White 30 30 Asian- American Asian- American 20 20 Hispanic Hispanic 10 10 African- American African- American 0 0 2010 ’12 ’14 ’12 ’16 ’14 2010 ’16 Applicants % 50 40 White 30 Asian- American 20 Hispanic 10 African- American 0 ’14 ’12 2010 ’16 Admitted students % 70 60 All other, including white* 50 40 30 Asian- American 20 Hispanic 10 African- American 0 2010 ’14 ’16 ’12

Students who qualify for these letters are twice as likely to be admitted as students who don’t qualify, according to a handbook provided to Harvard’s alumni interviewers.

William Fitzsimmons, Harvard’s admissions dean since 1986, defended the policy by saying the letters to white students in more rural states help the school recruit from areas where students may be less aware of Harvard. “We do everything we can to reach out to a much broader range of people,” he testified.

Mr. Fitzsimmons, 74 years old, said the lower thresholds for underrepresented minorities take into consideration how the “rather stark economic differences and opportunities” those students face may affect their ability to score higher on standardized tests.

A lawyer for the plaintiffs said a white student in a state like Nevada would receive a Harvard recruitment letter if he or she scored 1310 on the PSAT, while an Asian student in the same state with the same score wouldn’t, amounting to what he called racial discrimination. Mr. Fitzsimmons denied the allegation.

The trial stems from a lawsuit filed in 2014 by Students for Fair Admissions, a nonprofit whose members include Asian-Americans rejected by Harvard. The group is run by conservative legal strategist Edward Blum, who has funded other challenges to racial preferences in college admissions.

At opening statements Monday, the two sides sharply disagreed—even about the central dispute of the trial.

Adam Mortara, a partner at Bartlit Beck Herman Palenchar & Scott LLP who is representing the plaintiffs, said the future of affirmative action and the benefits of diversity weren’t on trial, noting that the plaintiffs supported diversity on campus.

“This trial is about what Harvard…is doing to Asian-American applicants and how far Harvard has gone in its zeal to use race in its admissions process,” Mr. Mortara said.

William Lee, a WilmerHale partner representing Harvard, said the plaintiffs’ stated purpose is to “eliminate all consideration of race in college admissions,” which he called an attack on diversity in higher education. Race is never the reason a student is admitted or rejected from Harvard, he said, adding the school considers race as one of many factors, in line with Supreme Court precedents.

“After four years of litigation,” Mr. Lee said, “the plaintiff cannot prove that Harvard discriminates against Asian-American applicants.”

Lawyers for a group of current and former Harvard students who support the university said in their opening statement that any disadvantage Asian-Americans face in the admissions process isn’t due to the consideration of race, but due to preferences, such as for legacies and recruited varsity athletes, that primarily help white applicants.

The trial is expected to delve into the magnitude of those preferences. Mr. Fitzsimmons acknowledged Monday that legacy and recruited athlete applicants are much more likely to get in than other applicants. But both sides dispute whether those preferences are the only explanation for the different acceptance rates for white and Asian-American applicants.

Vietnamese-American students at Harvard are expected to testify about how the consideration of race helped them in the admissions process. The students’ lawyers say Harvard seeks to cultivate diversity within each racial group, as the label “Asian-American” encompasses a range of ethnicities and backgrounds.

Later in the trial, Mr. Fitzsimmons will be asked to explain why Asian-American applicants receive the highest academic and extracurricular scores among any racial group, but the lowest “personal” scores, a factor in Harvard’s admissions that assesses personality traits such as likability, courage and kindness.

He will also be asked why he didn’t pursue further investigation of internal 2013 reports that found being Asian-American decreased an applicant’s chance of admission.

Harvard has said in court filings that the lower personal rating can be explained by observable and “unobservable factors” seen in teacher recommendations, essays and interviews, and that the internal reports were preliminary and incomplete.

A judge, not a jury, will decide whether the trial testimony proves Harvard’s admissions practices violate federal civil-rights law. It isn’t known when U.S. District Judge Allison Burroughs, an Obama appointee, will issue her written ruling after the trial’s conclusion, but it could take several months. Her decision is likely to be appealed.

Throughout the trial, the judge will hear from current and former admissions officers; Drew Gilpin Faust, the university’s former president; and Rakesh Khurana, dean of the undergraduate Harvard College.

Write to Nicole Hong at nicole.hong@wsj.com and Melissa Korn at melissa.korn@wsj.com