The social experience of transitioning to a 4-year university varies widely among students. Some attend with few or no prior contacts or acquaintances from their hometown; others attend with a large network of high school alumni. Using a sample (N = 43,240) of undergraduates spanning 7.5 years at a public university, we examine what factors predict high school peer prevalence (HSPP) on campus and whether HSPP predicts college achievement above and beyond such factors. Analyses found that HSPP was predicted by variables associated with societal privilege (e.g., being White, continuing generation). Above and beyond these variables, HSPP independently predicted higher grades in gateway STEM (science, technology, engineering, and mathematics) courses and, among first-generation college students, higher retention. The role of HSPP in fostering equity and inequity is discussed. A preprint of this article is available at https://psyarxiv.com/xhpuc/.