In Canada, about 6% of 15-year-olds attend a private school. Although some provincial governments subsidize a portion of the costs, parents must still pay for their child to attend. Differences in the academic outcomes of children who attend public and private schools continue to attract public attention. A central issue is the extent to which better outcomes among private school students are attributable to students' socioeconomic characteristics or to differences in school resources and practices. This article examines a survey sample of 7,142 15-year-olds who were registered in Grade 10 in public and private high schools and subsequently followed until age 23. The follow-up period facilitates analysis of a broad range of academic outcomes, including test scores at age 15 and educational attainment by age 23. The study focuses on: (1) the gaps in academic outcomes by school sector; (2) differences in factors associated with academic outcomes (socio-economic characteristics, province of school attendance, school resources and practices, and peer characteristics) by school sector; and (3) the proportion of the gaps in academic outcomes that are associated with these factors. Three conclusions emerge from this study. First, on average, students who attended private high schools scored higher on academic tests at age 15, and had higher levels of educational attainment by age 23, than did students who attended public high schools. Second, the characteristics of public and private high school students were different. Compared with public high school students, those in private schools generally came from families with a higher socio-economic status (as measured by parental income and education), and were more likely to be surrounded by peers whose parents had attended university. As well, public and private schools were concentrated in different provinces, and thus, may have followed different curricula. However, few differences in resources and practices across school sectors were observed. Finally, two factors accounted for a substantial portion of the differences between the public and private sectors in all of the academic outcomes examined: socio-economic characteristics and peers. The province of school attendance accounted for a substantial portion of the differences in academic outcomes at the high school level (test scores and graduation rates), but less at the postsecondary level. School resources and practices played little to no role in the differences in each academic outcome observed.