BOSTON—Harvard’s admissions dean testified Tuesday that weaker teacher and guidance-counselor recommendations are one reason why Asian-American applicants as a group score lower than white applicants in the “personal rating” portion of the school’s admissions process.

The rating, which assesses an applicant’s personal qualities, has been a central focus for the plaintiffs in a trial that began Monday accusing Harvard of intentionally discriminating against Asian-Americans. Harvard’s own data show Asian-American applicants...