Using data from the 2003 National Assessment of Educational Progress, this analysis compared mathematics achievement in public, charter, and major types of private schools to examine whether disparities in achievement are due to differences in school performance or student demographics in various sectors. Hierarchical linear models were used to control for student- and school-level demographic characteristics. The analysis indicated that the relatively high raw scores of private schools were more than accounted for by student demographics. In fact, after demographic differences had been controlled, the private school advantage disappeared and even reversed in most cases. These findings raise questions about the basis of reform models that seek remedies in parental choice, autonomy, competition, and other attributes associated with the private school sector. (Contains 4 tables, 2 figures and 29 notes.)