Background: Private schools have a big part in the educational landscape in the United States, both in terms of the number of students they educate and in the number of resources they command. That said, despite expectations to increase equity and access to opportunity for all students, little is known about racial, gender, or socioeconomic achievement trends in this setting. Unlike public schools, there is very limited national-level achievement data infrastructure for private schools. Analyses of individual schools can begin to illuminate trends in particular contexts. Purpose of Study: This study examines how achievement differs by race, gender, and socioeconomic status in one private school in the United States, Gardenview Preparatory School (GPS; a pseudonym). Setting: GPS is a traditional private school serving students pre-kindergarten to 12th grade, located in a wealthy, predominantly White suburban area of a larger metropolitan region. Leadership in the school have attempted to prioritize diversity, equity, and inclusion (DEI) work. One of their attempts was to seek a research collaboration with the authors of this article. Participants: The primary focus of this article included a quantitative sample of all students graduating from GPS between the years of 2010 to 2017 (N=1216). Research Design: While the larger research collaboration included mixed methods, this article presents multivariable regression models using secondary data. Significant features of the regression models include (1) achievement measures at both admissions to GPS and graduation from GPS, (2) multiple measures of achievement at graduation, and (3) intersectional analyses. Findings: A key takeaway from this analysis is that demographics are not largely predictive of admissions test performance at GPS. A second key takeaway, however, is that once GPS students reached high school graduation, larger and more widespread racial, gendered, and socioeconomic disparities emerged. The third major takeaway from the analysis is that intersections of race and gender do impact achievement upon graduation from GPS. Conclusions: This study is one of the first of its kind to use advanced multivariable regression to map achievement disparities--at multiple timepoints in a student's career and using multiple measures of achievement--in a traditional, private school. Without understanding the existence, nature, and extent of achievement disparities, stakeholders cannot act intentionally to promote equity in private schools and beyond.