Interviewing 31 individuals at a public research institution we examine students' assumptions about compositional social class diversity (CSCD) on campus and the ways various dimensions of climate for social class diversity interplay with students' awareness of CSCD. Findings reveal the assumptions that guide students' interpretation of CSCD on campus and the difficulty with knowing if a campus is diverse. We show that not only numerical demographic representation but also students' perceptions of diversity are important for understanding campus CSCD, given social class is often hard to define and observe on campus. Implications for higher education are discussed.